ARM: gic: Use cpu pm notifiers to save gic state

When the cpu is powered down in a low power mode, the gic cpu
interface may be reset, and when the cpu cluster is powered
down, the gic distributor may also be reset.

This patch uses CPU_PM_ENTER and CPU_PM_EXIT notifiers to save
and restore the gic cpu interface registers, and the
CPU_CLUSTER_PM_ENTER and CPU_CLUSTER_PM_EXIT notifiers to save
and restore the gic distributor registers.

Original-author: Gary King <gking@nvidia.com>
Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Tested-and-Acked-by: Shawn Guo <shawn.guo@linaro.org>
Tested-by: Vishwanath BS <vishwanath.bs@ti.com>