mtd: maps: add mtd-ram support to physmap_of

Use physmap_of to access RAMs as mtd and add documenation for it. This approach
is a lot less intrusive as adding an of-wrapper around plat-ram.c. As most
extensions of plat-ram.c (e.g. custom map-functions) can't be mapped to the
device tree anyhow, extending physmap_of seems to be the cleanest approach.

Tested with a phyCORE-MPC5121e.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: Vitaly Wool <vwool@ru.mvista.com>
Cc: Artem Bityutskiy <dedekind@infradead.org>
Cc: Ken MacLeod <ken@bitsko.slc.ut.us>
Cc: Albrecht Dreß <albrecht.dress@arcor.de>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

drivers/mtd/maps/physmap_of.c

@@ -361,6 +361,10 @@ static struct of_device_id of_flash_match[] = {
 		.compatible	= "jedec-flash",
 		.data		= (void *)"jedec_probe",
 	},
+	{
+		.compatible     = "mtd-ram",
+		.data           = (void *)"map_ram",
+	},
 	{
 		.type		= "rom",
 		.compatible	= "direct-mapped"