PM: Use spinlock instead of mutex in clock management functions

The lock member of struct pm_clk_data is of type struct mutex,
which is a problem, because the suspend and resume routines
defined in drivers/base/power/clock_ops.c cannot be executed
with interrupts disabled for this reason.  Modify
struct pm_clk_data so that its lock member is a spinlock.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Magnus Damm <damm@opensource.se>