ARM: dts: sun6i: hummingbird: Add AXP221 PMIC device node

The Hummingbird A31 has an AXP221 PMIC hooked up to the
P2WI controller.

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