arm64: kernel: implement fpsimd CPU PM notifier

When a CPU enters a low power state, its FP register content is lost.
This patch adds a notifier to save the FP context on CPU shutdown
and restore it on CPU resume. The context is saved and restored only
if the suspending thread is not a kernel thread, mirroring the current
context switch behaviour.

Change-Id: I7f340bd7f9747a3f1e8af043a30f5c3d8ec7d98f
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Git-commit: fb1ab1ab3889fc23ed90e452502662311ebdf229
Git-Repo: git://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/


[abhimany@codeaurora.org: resolve minor merge conflict]
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>