ARM: dt: Add binding for Tegra PMC

The Tegra PMC (Power Management Controller) interfaces with an external
PMU (Power Management Unit), and controls wake-up from sleep modes.

This initial binding is the bare minimum required to control the PMC's
inversion of the PMU's interrupt signal.

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