This supports CPU core power down on each CPU when CPU idle. When CPU go
into this state, it saves it's context and needs a proper configuration
in flow controller to power gate the CPU when CPU runs into WFI
instruction. And the CPU also needs to set the IRQ as CPU power down idle
wake up event in flow controller.

Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: NJoseph Lo <josephl@nvidia.com>
Signed-off-by: NStephen Warren <swarren@nvidia.com>