mtd: plat-ram: Replace manual resource management by devm

Driver contains unsuitable request_mem_region() and
release_resource() calls.

The patch switches manual resource management by devm interface for
readability and error-free simplification.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Anton Vasilyev <vasilyev@ispras.ru>
Suggested-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Richard Weinberger <richard@nod.at>