mtd: maps: physmap: Use devm_ioremap_resource()

Use devm_ioremap_resource() to replace the devm_request_mem_region() +
devm_ioremap() combination.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>