mfd: twl4030-power: Add a configuration to turn off oscillator during off-idle

Some oscillators can be turned off during off-idle saving few
a little bit power at the cost of the oscillator start up
latency.

If you board can do this, you can now enable it by using the
ti,twl4030-power-idle-osc-off compatible flag.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>