spi: spi-mem: allow specifying whether an op is DTR or not

Each phase is given a separate 'dtr' field so mixed protocols like
4S-4D-4D can be supported.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20200623183030.26591-2-p.yadav@ti.com


Signed-off-by: Mark Brown <broonie@kernel.org>