mmc: tegra: Set calibration pad voltage reference

Configure the voltage reference used by the automatic pad drive strength
calibration procedure. The value is a magic number from the TRM.

Signed-off-by: Aapo Vienamo <avienamo@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>