ARM: tegra: prepare powergate.c for multiple variants

Prepare the powergating code for other Tegra variants which have a different
number of powerdomains.

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