PM: Reference counting of power.subsys_data

Since the power.subsys_data device field will be used by multiple
filesystems, introduce a reference counting mechanism for it to avoid
freeing it prematurely or changing its value at a wrong time.

Make the PM clocks management code that currently is the only user of
power.subsys_data use the new reference counting.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

diff --cc drivers/base/power/Makefile
Simple merge

diff --cc drivers/base/power/clock_ops.c
Simple merge

diff --cc drivers/base/power/common.c
Simple merge

diff --cc include/linux/pm.h
Simple merge