dt-bindings: mfd: axp20x: Introduce bindings for AXP813

The X-Powers AXP813 is a PMIC designed to be paired with Allwinner's
A83T SoC. There is also an AXP818, which is paired with the H8 SoC.

The two models seem to be identical, apart from the external markings.

This patch introduces the basic mfd and regulator bindings for the
AXP813.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>