mtd: physmap_of: Add non-obsolete map_rom probe

Previously, the only way to map a NOR device as a simple ROM was to
use the obsolete "direct-mapped" compatible binding (which further
requires device_type = "nor" and probe-type = "NOR" properties).

This patch adds an "mtd-rom" compatible binding to the "map_rom"
probe type.

Signed-off-by: Aaron Sierra <asierra@xes-inc.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>