arm64: kernel: implement fpsimd CPU PM notifier

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.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
(cherry picked from commit fb1ab1ab3889fc23ed90e452502662311ebdf229)
Signed-off-by: Mark Brown <broonie@linaro.org>