mtd: spi-nor: Add Octal DDR mode support for Micron mt35x flash

Add support for Octal DDR mode to read data from Micron mt35x flash. For
now enter stateful Octal DDR mode just before reading data from flash
and exits the mode after reading. Benefit of using DDR mode for write is
limited as most time is actually spent in waiting for write to complete
than actual write itself.
This also allows us to be compatible with ROM code which expects flash
to in normal mode (and not Octal DDR mode)

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