ARM: mx51: clean up mx51 header

This makes the header more look like the other ones, i.e.

 - sort #defines by value
 - use lowercase hex constants
 - use a consistently named header guard

Acked-by: Jason Wang <jason77.wang@gmail.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>