soc/tegra: pmc: Add support for simple wake events

Simple wake events are neither mapped to GIC interrupts nor have an
associated GPIO line. They are close to GPIO-backed wake events in that
the IRQ hierarchy processing needs to stop at the PMC level, but since
there is no dedicated GPIO line for them, let's turn them into a
separate type.

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