ARM: tegra: beaver: Allow SD card voltage to be changed

This allows to switch the card signal voltage level to 1.8 V, which is
needed for any ultra high speed modes to work.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>