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: Jagan Teki <jagan@amarulasolutions.com>