mtd: spi-nor: Add spansion_post_sfdp_fixups()

Add a spansion_post_sfdp_fixups() function to fix the erase opcode,
erase sector size and set the SNOR_F_4B_OPCODES flag.
This way, all spansion related quirks are placed in the
spansion_post_sfdp_fixups() function.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Vignesh Raghavendra <vigneshr@ti.com>