Device Tree Bindings for Power Controller on MediaTek PMIC

The power controller which could be found on PMIC is responsible for externally
powering off or on the remote MediaTek SoC through the circuit BBPU.

Required properties:
- compatible: Should be one of follows
       "mediatek,mt6323-pwrc": for MT6323 PMIC

Example:

       pmic {
               compatible = "mediatek,mt6323";

               ...

               power-controller {
                       compatible = "mediatek,mt6323-pwrc";
               };
       }
