mmc: mmc_mxc: Allow selection only for the correct platforms

Currently MMC_MXC driver can be selected by all i.MX devices.

Restrict its use only for the appropriate processors.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>