ARM: mx5/mm: move i.MX50 mm stuff into mm.c

i.MX50 is similar enough to i.MX51/53 to handle it in a single file.
No functional change.

Signed-off-by: Jason Liu <jason.hui@linaro.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>