mtd: spinand: winbond: Add support for continuous reads on W35NxxJW

mtd: spinand: winbond: Add support for continuous reads on W35NxxJW

W35N{01,02,04}JW support being read continuously under certain
circumstances. A bit must be set in their configuration register, and
a specific read from cache operation, a bit shorter than usual because
it no longer requires the address cycles, must be used for the occasion.

Setting the "enable" bit is already supported by the core, aside from
the subtlety of making sure the HFREQ bit is also set in octal DTR mode
above 90MHz. However, handling two different read from cache templates
involves creating a list of read from cache variants adapted the
continuous reads, ie. without address cycles.

Unfortunately, these operations, despite being very close to their
original read from cache cousins, are often unsupported by smart SPI
controller drivers because reading from cache historically allowed
changing the offset at which the host would start by providing a 2-byte
column address. In order to prevent issues with this, it has been
decided to implement these variants with a single "ignored" address byte
(respectively two in the octal DTR case), further reducing the amount of
dummy cycles needed before the first bit of data.

Enabling continuous reads has a side effect: the ECC status register now
may also return the value b11, which means that more than 1
uncorrectable error happened during the read. This non standard
behaviour requires to re-implement, almost identically the "get ECC"
helper from the core, with just an extra case for this value (it is
prefixed "w25w35nxxjw" because all these chips have the same behaviour).

Speed gain is substantial, see below. The flash_speed -C benchmark has
been run on a TI AM62A7 LP SK with CPU power management disabled,
mounted with a W35N01JW chip.

1S-8S-8S:

     1 page read speed is 15058 KiB/s
     2 page read speed is 15058 KiB/s
     3 page read speed is 16800 KiB/s
     4 page read speed is 17066 KiB/s
     5 page read speed is 18461 KiB/s
     6 page read speed is 18461 KiB/s
     7 page read speed is 19384 KiB/s
     8 page read speed is 19692 KiB/s
     9 page read speed is 19384 KiB/s
     10 page read speed is 20000 KiB/s
     11 page read speed is 20000 KiB/s
     12 page read speed is 20000 KiB/s
     13 page read speed is 20800 KiB/s
     14 page read speed is 20363 KiB/s
     15 page read speed is 20000 KiB/s
     16 page read speed is 19692 KiB/s
     32 page read speed is 19692 KiB/s
     64 page read speed is 19692 KiB/s

8D-8D-8D:

     1 page read speed is 23272 KiB/s
     2 page read speed is 23272 KiB/s
     3 page read speed is 28000 KiB/s
     4 page read speed is 32000 KiB/s
     5 page read speed is 34285 KiB/s
     6 page read speed is 34285 KiB/s
     7 page read speed is 36000 KiB/s
     8 page read speed is 36571 KiB/s
     9 page read speed is 36000 KiB/s
     10 page read speed is 34285 KiB/s
     11 page read speed is 36666 KiB/s
     12 page read speed is 40000 KiB/s
     13 page read speed is 41600 KiB/s
     14 page read speed is 37333 KiB/s
     15 page read speed is 40000 KiB/s
     16 page read speed is 36571 KiB/s
     32 page read speed is 42666 KiB/s
     64 page read speed is 42666 KiB/s

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
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Not all configurations have been tested/validated yet.