ARM: tegra: Add a simple PMC driver

This PMC driver is enough to parse the nvidia,invert-interrupt property
from device tree, and configure the PMC's to honor that.

In the future, this file could expand to centralize all other PMC accesses
within the mach-tegra code.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>