mx23: register clocks for mmc

Commit 47babe69 (mxs: dynamically allocate mmc device) added the ssp
setup and mmc clocks for mx23/28, but forgot to register the mmc clocks
on mx23.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Acked-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>