mtd: spinand: winbond: Convert W35N specific operation to SPINAND_OP()

Winbond W35N* chips require a vendor specific operation to write their
VCR register (a configuration register, typically used for tuning the
number of dummy cycles and switching to a different bus
interface). Instead of defining this op only in the function that needs
it, hiding it from the core, make it a proper define like all other
spi-mem operations, and implement the necessary spinand_fill_*_op()
helper to make the SPINAND_OP() macro work. This way we can use it from
any function without any extra handling outside of this helper when we
will convert the core to support octal DDR busses.

Reviewed-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>