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>