Only allow identity memory mapping

The code has the assumption that virtual addresses are mapped to the
same physical address but the mm function would allow this assumption to
be broken. Only offering identity mapping function avoids this problem
by enforcing assumption.

Non-identity mapping can still be added later if there proves to be a
need for it.

Change-Id: Ie3db41e1582c72d880275acdb48cabefe05b3ba0
diff --git a/src/main.c b/src/main.c
index 7938fec..72815d7 100644
--- a/src/main.c
+++ b/src/main.c
@@ -68,8 +68,8 @@
 	     params.initrd_end - 1);
 
 	/* Map initrd in, and initialise cpio parser. */
-	if (!mm_map(params.initrd_begin, params.initrd_end, params.initrd_begin,
-		    MM_MODE_R)) {
+	if (!mm_identity_map(params.initrd_begin, params.initrd_end,
+			     MM_MODE_R)) {
 		panic("unable to map initrd in");
 	}