mfd: Add power control interface for pm8606 chip

The reference group and internal oscillator are shared by sub-devs
like led, backlight and vibrator in PM8606 chip. Now introduce a
voting mechanism to enable/disable it.

Add pm8606_osc_enable() and pm8606_osc_disable() interface and
related defines to support this. This interface will be called by
vibrator led and backlight driver.The refernce group and internal
oscillator are enabled only when at least one of it's clients holds
it on or disabled only all the clients don't use it any more based
on the above mechanism.

Signed-off-by: Jett.Zhou <jtzhou@marvell.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>