soc/tegra: Implement Tegra186 PMC support

The power management controller on Tegra186 has changed in backwards-
incompatible ways with respect to earlier generations. This implements a
new driver that supports inversion of the PMU interrupt as well as the
"recovery", "bootloader" and "forced-recovery" reboot commands.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>