ARM: tegra: Export tegra_powergate_remove_clamping()

Drivers can use the tegra_powergate_remove_clamping() API during
initialization. In order to allow such drivers to be built as modules,
export the symbol.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>