mtd: spi-nor: swp: Add support for the complement feature

mtd: spi-nor: swp: Add support for the complement feature

The current locking implementation allows to select a power of two
number of blocks, which is going to be the protected amount, as well as
telling whether this is the data at the top (end of the device) or the
bottom (beginning of the device). This means at most we can cover half
of the device or the entire device, but nothing in between.

The complement feature allows a much finer grain of configuration, by
allowing to invert what is considered locked and unlocked.

Add support for this feature. The only known position for the CMP bit is
bit 6 of the configuration register.

The locking and unlocking logics are kept unchanged if the CMP bit is
unavailable. Otherwise, once the regular logic has been applied, we
check if we already found an optimal configuration. If not, we try with
the CMP bit set. If the coverage is closer to the request, we use it.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Pratyush Yadav <pratyush@kernel.org>