ARM: dts: sunxi: Add dtsi for AXP209 PMIC

The AXP209 PMIC is used with some Allwinner SoCs. This patch adds
a dtsi file listing all the regulator nodes. The regulators are
initialized based on their device node names.

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