power: pmic: sunxi: introduce generic SPL AXP DC/DC driver

power: pmic: sunxi: introduce generic SPL AXP DC/DC driver

So far we had a separate driver file for each AXP PMIC chip that we need
to support in the SPL. The code in there was largely similar, but
differed in many details.

Based on the idea of the DM AXP driver, introduce a data structure to
describe each regulator in a compact way. This is a simplified version
of the struct used in the DM driver, as we don't need to support the full
voltage range and not every regulator in the SPL.
For now we only support the DC/DC buck converters, since that's what we
need the SPL to configure, mostly. Also we get rid of the regulator name,
and hardcode the regulator number by its position in the array (first is
DCDC1, second is DCDC2, etc). We also drop support for the value table,
we ideally won't need that for the subset of regulators required.
At the end each regulator is described by a 10 bytes struct, so we avoid
blowing up the SPL footprint, but still can use generic code.

Each chip is supposed to be described separately, and protected by
ifdef's, to only build in the regulators needed for a particular board.
We also describe the bits to help identifying the AXP chip, and the
shutdown details in that section.

Add a generic driver, that exports axp_set_dcdc<x>() functions to set up
the buck converters. For now this just contains the bits for the (new)
AXP717, but it's not wired up anywhere yet.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>