mtd: spi-nor: use spi-mem dirmap API

Make use of the spi-mem direct mapping API to let advanced controllers
optimize read/write operations when they support direct mapping.

Based on the original patch by Boris Brezillon
<boris.brezillon@bootlin.com>.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>