rk805: Add a regnode_init method

This method will set the desired voltaged based on values in the DTS.
It will not enable the regulator, this is the job of either a consumer
or regnode_set_constraint SYSINIT if the regulator is boot_on or always_on.

Reviewed by: mmel
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D23216