Add MMC Framework

Here's a new framework (based roughly off the linux one) for managing
MMC controllers.  It handles all of the standard SD/MMC transactions,
leaving the host drivers to implement only what is necessary to
deal with their specific hardware.

This also hooks the infrastructure into the PowerPC board code
(similar to how the ethernet infrastructure now hooks in)

Some of this code was contributed by Dave Liu <daveliu@freescale.com>

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