PM: clk: remove kernel-doc warning

Remove make W=1 warning:

drivers/base/power/clock_ops.c:148: warning: expecting prototype for
pm_clk_enable(). Prototype was for __pm_clk_enable() instead

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>