mtd: spi-nor: Add support for statefull Octal IO mode

xSPI compliant flashes support only 1 bit mode or 8 bit DDR mode.
Entering 8 bit DDR mode would require configuring certain registers in
flash in 1 bit mode. This procedure varies across flash vendor. Also,
different vendors require different number of wait cycles (dummy bytes)
in Octal DDR mode. Therefore provide two callbacks for flashes: change_op()
and adjust_op(). change_op() can be used for switching to Octal mode and
adjust_op() to adjust protocol specific settings for OSPI.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>