When the physmap_of_core.c code was merged into physmap-core.c the
ability to use MTD_PHYSMAP_OF with only MTD_RAM selected was lost.
Restore this by adding MTD_RAM to the dependencies of MTD_PHYSMAP.

Fixes: commit 642b1e8d ("mtd: maps: Merge physmap_of.c into physmap-core.c")
Cc: <stable@vger.kernel.org>
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Hamish Martin <hamish.martin@alliedtelesis.co.nz>
Signed-off-by: Richard Weinberger <richard@nod.at>