mtd: spi-nor: core: avoid odd length/address writes in 8D-8D-8D mode

On Octal DTR capable flashes like Micron Xcella the writes cannot start
or end at an odd address in Octal DTR mode. Extra 0xff bytes need to be
appended or prepended to make sure the start address and end address are
even. 0xff is used because on NOR flashes a program operation can only
flip bits from 1 to 0, not the other way round. 0 to 1 flip needs to
happen via erases.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Signed-off-by: Luke Wang <ziniu.wang_1@nxp.com>
Signed-off-by: Pratyush Yadav <pratyush@kernel.org>
Link: https://lore.kernel.org/r/20250708091646.292-2-ziniu.wang_1@nxp.com