arm64: tegra210: define arch option for t210

arm64: tegra210: define arch option for t210

This change also adds stub implementation for tegra_pd_in_idle. It's
added in board-exuma.c for now, but this will be reverted once
grenada specific code is ready.

Change-Id: Ia7dc136f67fd7af422262c5b7e203823e2087950
Signed-off-by: Bo Yan <byan@nvidia.com>
Reviewed-on: http://git-master/r/268604
Reviewed-by: Seshendra Gadagottu <sgadagottu@nvidia.com>
Tested-by: Seshendra Gadagottu <sgadagottu@nvidia.com>