mtd: spi-nor: Add support for Octal DDR mode

Add framework level support required 8 bit DDR mode. Drop 8 bit SDR mode
as its only supported by Micron and is not required to be supported as per
xSPI specification.
Note that that opcodes are Micron specific and does not cover all flash
vendors

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