spi: spi-mem: Introduce a capability structure

Create a spi_controller_mem_caps structure and put it within the
spi_controller structure close to the spi_controller_mem_ops
strucure. So far the only field in this structure is the support for dtr
operations, but soon we will add another parameter.

Also create a helper to parse the capabilities and check if the
requested capability has been set or not.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20220127091808.1043392-2-miquel.raynal@bootlin.com