arm: odroid: pmic77686: allow buck voltage settings

Allow to set the buck voltage for the max77686.
This will be used to reset the SMC LAN9730 ethernet on the odroids.

Signed-off-by: Suriyan Ramasami <suriyan.r@gmail.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>