mmc: Tegra: Add SD bus power/voltage function and MMC pad init call.

Tegra30 requires the SD Bus Voltage & Power bits be set in the SD
Power Control register. Tegra20 works w/o them set, but do it anyway
for those SoCs as it's part of the SD spec. Also call a common
board pad init routine (pad_init_mmc) in mmc_reset(), used by
Tegra30 only for now.

Note that Tegra20 SD/MMC HW differs enough from Tegra20 that a
new compatible entry is used in the fdt compat_names/id tables.

Signed-off-by: Tom Warren <twarren@nvidia.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>