The flow controller would take care the power sequence when CPU idle in
powered-down mode. It powered gate the CPU when CPU runs into WFI
instruction. And wake up the CPU when event be triggered.

The sequence is below.
* setting wfi bitmap for the CPU as the halt event in the
  FLOW_CTRL_CPU_HALT_REG to monitor the CPU running into WFI,then power
  gate it
* setting IRQ and FIQ as wake up event to wake up CPU when event triggered
Signed-off-by: Joseph Lo <josephl@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>