Add Runtime PM support to the TMU driver.

The hardware device is enabled as long as the clocksource
or the clockevent portion of the driver is used.
Signed-off-by: Magnus Damm <damm@opensource.se>
Acked-by: John Stultz <johnstul@us.ibm.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>