Previously, the A64/H5 and H6 platforms' PMIC setup code was entirely
independent. However, some H6 boards also need early regulator setup.

Most of the register interface and all of the device tree traversal code
can be reused between the AXP803 and AXP805. The main difference is the
hardware bus interface, so that part is left to the platforms. The
remainder is moved into a driver.

I factored out the bits that were obviously specific to the AXP803;
additional changes for compatibility with other PMICs can be made as
needed.

The only functional change is that rsb_init() now checks the PMIC's chip
ID register against the expected value. This was already being done in
the H6 version of the code.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Change-Id: Icdcf9edd6565f78cccc503922405129ac27e08a2