spi: mxic: Add support for direct mapping

Implement the ->dirmap_create() and ->dirmap_read/write() hooks to
provide a fast path for read and write accesses.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Tested-by: Zhengxun Li <zhengxunli@mxic.com.tw>
Reviewed-by: Zhengxun Li <zhengxunli@mxic.com.tw>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20220127091808.1043392-13-miquel.raynal@bootlin.com