Eliminated arch-specific mmc header requirement

The current MMC infrastructure relies on the existence of an
arch-specific header file.  This isn't necessary, and a couple
drivers were forced to implement dummy files to meet this requirement.
Instead, we move the stuff in those header files into a more appropriate
place, and eliminate the stubs and the #include of asm/arch/mmc.h

Signed-off-by: Andy Fleming <afleming@freescale.com>