clocksource: sh_tmu: Runtime PM support

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>