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>
diff --git a/drivers/mtd/maps/physmap_of.c b/drivers/mtd/maps/physmap_of.c
index 63d82da..c1d21cb 100644
--- a/drivers/mtd/maps/physmap_of.c
+++ b/drivers/mtd/maps/physmap_of.c
@@ -340,6 +340,10 @@
 		.data           = (void *)"map_ram",
 	},
 	{
+		.compatible     = "mtd-rom",
+		.data           = (void *)"map_rom",
+	},
+	{
 		.type		= "rom",
 		.compatible	= "direct-mapped"
 	},