Add of_regulator_get_optional() and Fix MTK Power

Merge series from Chen-Yu Tsai <wenst@chromium.org>:

At ELCE, Sebastian told me about his recent work on adding regulator
supply support to the Rockchip power domain driver [2], how the MediaTek
driver has been using the existing devm_regulator_get() API and
reassigning different device nodes to the device doing the lookup, and
how the new of_regulator_get_optional() is the proper fit for this.

Patch 1 adds a new of_regulator_get_optional() function to look up
regulator supplies using device tree nodes.

Patch 2 adds a devres version of the aforementioned function at
Sebastian's request for the two power domain drivers.