PM / clock_ops: make __pm_clk_enable more generic

Now there are two places in code which do the same things,
so allow __pm_clk_enable() to accept pointer on pm_clock_entry
structure as second parameter instead of pointer on clock and
remove duplicated code.

Also, updated function intended to be used by following patch.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>