spi: cadence-quadspi: Implement a simple supports_op hook

The default SPI MEM supports_op hook rejects DTR ops by default. Add a
simple supports_op hook that very closely imitates the SPI MEM one. It
will be extended in later commits to allow DTR ops.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20201222184425.7028-6-p.yadav@ti.com
Signed-off-by: Mark Brown <broonie@kernel.org>