tegra2: Move MMC clock initialization into MMC driver
This centralizes knowledge of MMC clocking into the MMC driver. This also
removes clock setup from the board files, which will simplify later changes
that modify the Harmony board to support the correct set of MMC controllers.
Signed-off-by: Stephen Warren <swarren@nvidia.com> Cc: Andy Fleming <afleming@gmail.com> Acked-by: Simon Glass <sjg@chromium.org> Tested-by: Simon Glass <sjg@chromium.org>