AMD: Uniformly define MSRs for TOP_MEM and TOP_MEM2

Make the build tolerate re-definitions.

Change-Id: Ia7505837c70b1f749262508b26576e95c7865576
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/8609
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/cpu/amd/car/cache_as_ram.inc

@@ -468,8 +468,8 @@ all_mtrr_msrs:
 	.long	IORRMask_MSR(1)
 
 	/* Top of memory MTRR MSRs */
-	.long	TOP_MEM_MSR
-	.long	TOP_MEM2_MSR
+	.long	TOP_MEM
+	.long	TOP_MEM2
 
 	.long	0x000 /* NULL, end of table */