arm64: rockchip: Add driver for the io domain

This driver configure the registers in the GRF according to the value
of the regulators for the platform.
Some IP can run with either 3.0V or 1.8V, if we don't configure them
correctly according to the external voltage used they will not work.
It's only done at boot time for now and might be needed at runtime for
IP like sdmmc.

Reviewed by: mmel
Tested On: RockPro64, Firefly-RK3399 (gonzo), AIO-3288 (mmel)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D22854