Add Richtek RTQ2208 SubPMIC support

Merge series from Alina Yu <alina_yu@richtek.com>:

This patch series adds support for RTQ2208 SubPMIC regulators.
The RTQ2208 is a multi-phase, programmable power management IC that
integrate with dual multi-configurable, synchronous buck converters
and two ldos. The bucks features wide output voltage range from 0.4V to 2.05V
and the capability to configure the corresponding power stages.