regulator: Add X-Powers AXP15060/AXP313a PMIC

Merge series from Andre Przywara <andre.przywara@arm.com>:

This patch series adds support for the X-Powers AXP15060 and AXP313a
PMIC, which are general purpose PMICs as seen on different boards with
different SOCs, mostly from Allwinner.

This is mostly a repost of the previous patches, combining both the
AXP313a and AXP15060 series, rebased on top of v6.4-rc3, and omitting
the patches that already got merged.
The first two patches are the successors of the AXP313a v10 post,
the third patch is based on Shengyu's AXP15060 v3 post.

There were no code changes, just some tiny context differences due to
the rebase, plus I added the newly gained tags.

As the DT bindings and the AXP15060 MFD part are already in the tree,
this is just completing support with the MFD part for the AXP313a, and
the regulator support for both PMICs.