ARM: tegra: Provide dummy powergate implementation

In order to support increased build test coverage for drivers, implement
dummies for the powergate implementation. This will allow the drivers to
be built without requiring support for Tegra to be selected.

This patch solves the following build errors, which can be triggered in
v3.13-rc1 by selecting DRM_TEGRA without ARCH_TEGRA:

drivers/built-in.o: In function `gr3d_remove':
drivers/gpu/drm/tegra/gr3d.c:321: undefined reference to `tegra_powergate_power_off'
drivers/gpu/drm/tegra/gr3d.c:325: undefined reference to `tegra_powergate_power_off'
drivers/built-in.o: In function `gr3d_probe':
drivers/gpu/drm/tegra/gr3d.c:266: undefined reference to `tegra_powergate_sequence_power_up'
drivers/gpu/drm/tegra/gr3d.c:273: undefined reference to `tegra_powergate_sequence_power_up'

Signed-off-by: Thierry Reding <treding@nvidia.com>
[swarren, updated commit description]
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>