mmc: mxcmmc: add mpc512x SDHC support

The SDHC controller on mpc512x is compatible with i.MX31 SDHC,
so the mxcmmc driver can be used on mpc512x, too. Extend the
driver to support mpc512x as well.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>