BBB: Add "mmc" command that interfaces to the onboard eMMC

[Jarielle Catbagan]

These series of patches add the "mmc" command which is the interface to the onboard eMMC on the BBB.
The "mmc" command is implemented with initialization, reading, and writing functionality.

Furthermore, the MMC0 and MMC1 clock enable are moved to initCPUio() in cpuio.c to ensure that the
MMC modules are enabled before they are accessed.