mtd: spinand: Use secondary ops for continuous reads

mtd: spinand: Use secondary ops for continuous reads

In case a chip supports continuous reads, but uses a slightly different
cache operation for these, it may provide a secondary operation template
which will be used only during continuous cache read operations.

From a vendor driver point of view, enabling this feature implies
providing a new set of templates for these continuous read
operations. The core will automatically pick the fastest variant,
depending on the hardware capabilities.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>