cpu/amd/car: Use define MSR_MCFG_BASE rather than hardcoded value

Change-Id: I0b40c9811115b204f1cae70546d236049c1b3d30
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/5431
Tested-by: build bot (Jenkins)
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>

src/cpu/amd/car/cache_as_ram.inc

@@ -160,7 +160,7 @@ CAR_FAM10_out:
    #else
 	#error "bad MMCONF_BUS_NUMBER value"
    #endif
-	movl	$(0xc0010058), %ecx
+	movl	$MSR_MCFG_BASE, %ecx
 	wrmsr
 #endif