arm64: kernel: add PM build infrastructure

This patch adds the required makefile and kconfig entries to enable PM
for arm64 systems.

The kernel relies on the cpu_{suspend}/{resume} infrastructure to
properly save the context for a CPU and put it to sleep, hence this
patch adds the config option required to enable cpu_{suspend}/{resume}
API.

In order to rely on the CPU PM implementation for saving and restoring
of CPU subsystems like GIC and PMU, the arch Kconfig must be also
augmented to select the CONFIG_CPU_PM option when SUSPEND or CPU_IDLE
kernel implementations are selected.

Change-Id: Idfe12b087b22b11f6eb87379615394081ebab7f0
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Git-commit: 166936bace056dfc11452d794209f39a5e9b0fb4
Git-Repo: git://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git


Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>