mmc: tegra: move pad_init_mmc() into MMC driver

pad_init_mmc() is performing an SoC-specific operation, using registers
within the MMC controller. There's no reason to implement this code
outside the MMC driver, so move it inside the driver.

Cc: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>