arm/tegra: cleanup tegra20 support

* add a dependency to ARCH_TEGRA_2x_SOC in Kconfig to all tegra20 based boards
  and TEGRA_PCI
* make powergating dependent on ARCH_TEGRA_2x_SOC
* remove dependency on ARCH_TEGRA_2x_SOC for clock.c

Signed-off-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Colin Cross <ccross@android.com>
Signed-off-by: Olof Johansson <olof@lixom.net>