mtd: allow CONFIG_MTD_PHYSMAP_OF also for CONFIG_MTD_RAM

Up to now mtd-ram devices described in device trees were only accessible
if mtd-flash or mtd-rom were also configured at linux configuration
time, because MTD_PHYSMAP_OF was only available if (MTD_CFI ||
MTD_JEDECPROBE || MTD_ROM).  Allow MTD_PHYSMAP_OF selection also
when only MTD_RAM is set.

Signed-off-by: Philippe De Muyter <phdm@macqel.be>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>