mtd: rfd_ftl: allow use of MTD_RAM for testing purposes

This allows the rfd_ftl to be used with the mtdram module, so we can
test different mtd sizes and test the rfd_ftl on machines without a
physical nor flash device.

Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210807214538.14484-2-sean@mess.org