Implement PM runtime functions and enable autosuspend.

Note that we save context in probe to avoid restoring invalid context
on the first resume. For system suspend, we have the new PM runtime
functions do most of the work.
Signed-off-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20211015104720.52240-5-tony@atomide.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>