PM: Move clock-related definitions and headers to separate file

Since the PM clock management code in drivers/base/power/clock_ops.c
is used for both runtime PM and system suspend/hibernation, the
definitions of data structures and headers related to it should not
be located in include/linux/pm_rumtime.h.  Move them to a separate
header file.

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