mtd: spi-nor-core: Do not start or end writes at odd address in DTR mode

mtd: spi-nor-core: Do not start or end writes at odd address in DTR mode

On DTR capable flashes like Micron Xcella the writes cannot start or end
at an odd address in DTR mode. Extra 0xff bytes need to be prepended or
appended respectively to make sure both the start and end addresses are
even.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Signed-off-by: Apurva Nandan <a-nandan@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Tested-by: Jonathan Humphreys <j-humphreys@ti.com>
Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>