mtd: spi-nor: introduce Double Transfer Rate (DTR) SPI protocols

This patch introduces support to Double Transfer Rate (DTR) SPI protocols.
DTR is used only for Fast Read operations.

According to manufacturer datasheets, whatever the number of I/O lines
used during instruction (x) and address/mode/dummy (y) clock cycles, DTR
is used only during data (z) clock cycles of SPI x-y-z protocols.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>