mmc: mxcmmc: DT support

Adding devicetree support for imx21-mmc and imx31-mmc. Based on generic
gpio helper functions by Guennadi and generic DMA devicetree bindings.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Chris Ball <cjb@laptop.org>