Trace clock_enable and clock_disable events.

We track changes in peripheral clock frequency with 'freq' events, but
not if the clock turns on or off. Logging clock_enable and clock_disable
events lets us get a better picture of the peripheral clocks from the
systraces.

Test: see clock_enable/disable events from 'adb shell atrace freq'.
Change-Id: I572a5cf87af8f37b22b5ffb726eb5c2c49886af1