N900 now seems to shut down the external oscillator when hitting
off-idle.

And Beagle XM seems to have OSC_EN pin connected to allow shutting
down the oscillator looking at the schematics. The oscillator
output is cut off in off-idle and you can monitor it from R56 on
the bottom side of the board near the power jack. Note that for
beagle we need to also enable the UART wake-up event, the others
have that enabled in earlier patches.

OMAP37XX EVM (TMDSEVM3730) does not seem to have twl4030 clken
pin connected, so there is no point trying to enable shutting
down of the oscillator on it for the extra latency it adds.
Signed-off-by: Tony Lindgren <tony@atomide.com>