backlight: Modified power control of pm860x backlight

Since several sub modules such as backlight, leds and vibrator depend
on the refernce group and internal oscillator in pm8606, so modified
the power control of backlight in pm8606 by unified interface.

Signed-off-by: Jett.Zhou <jtzhou@marvell.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>