ARM: mx51: fix naming of spi related defines

The names used now match the processor's reference manual.  Also remove
MXC from the interrupt defines to match the other imx platforms.

Acked-by: Wolfram Sang <w.sang@pengutronix.de>
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>