mtd: spi-nor-core: Add support for DTR protocol

Double Transfer Rate (DTR) is SPI protocol in which data is transferred
on each clock edge as opposed to on each clock cycle. Make
framework-level changes to allow supporting flashes in DTR mode.

Right now, mixed DTR modes are not supported. So, for example a mode
like 4S-4D-4D will not work. All phases need to be either DTR or STR.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>