spi: mxic: Provide a capability structure

This controller has DTR support, so advertize it with a capability now
that the spi-controller structure contains this new field. This will
later be used by the core to discriminate whether an operation is
supported or not, in a more generic way than having different helpers.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.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/20220127091808.1043392-5-miquel.raynal@bootlin.com