ARM: dts: sunxi: Add dtsi for AXP81x PMIC

The AXP81x family of PMIC is used with the Allwinner A83T and H8 SoCs.
This includes the AXP813 and AXP818. There is no discernible difference
except the labeling. The AXP813 is paired with the A83T, while the
AXP818 is paired with the H8.

This patch adds a dtsi file for all the common bindings for these two
PMICs. Currently this is just listing all the regulator nodes. The
regulators are initialized based on their device node names.

In the future this would be expanded to include power supplies and
GPIO controllers.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>