Tegra30: Add/enable Cardhu build (T30 reference board)

This build is stripped down. It boots to the command prompt.
GPIO is the only peripheral supported. Others TBD.

include/configs/tegra-common.h now holds common config options
for Tegra SoCs.

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