ARM: mxc: don't allow to compile together i.MX51 and i.MX53

The two SoCs have different PHYS_OFFSETs so it's not (yet) possible to
compile a single (working) kernel for these.

LAKML-Reference: 1302464943-20721-4-git-send-email-u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>