mtd: physmap_of: Release resources on error

During probe, if there was an error the memory region and the memory
map were not properly released.This can lead a system unusable if
deferred probe is in use.

Replace mem_request and map with devm_ioremap_resource

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>