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>
diff --git a/boards.cfg b/boards.cfg
index e4b0d44..b519bc4 100644
--- a/boards.cfg
+++ b/boards.cfg
@@ -290,6 +290,7 @@
 seaboard                     arm         armv7:arm720t seaboard          nvidia         tegra20
 ventana                      arm         armv7:arm720t ventana           nvidia         tegra20
 whistler                     arm         armv7:arm720t whistler          nvidia         tegra20
+cardhu                       arm         armv7:arm720t cardhu            nvidia         tegra30
 colibri_t20_iris             arm         armv7:arm720t colibri_t20_iris  toradex        tegra20
 u8500_href                   arm         armv7       u8500               st-ericsson    u8500
 snowball                     arm         armv7       snowball               st-ericsson    u8500