ARM: tegra: export tegra_powergate_is_powered()

Export tegra_powergate_is_powered(). This function will be used by the Tegra30
code to bringup secondary CPU cores.

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>