ARM: dts: sun6i: hummingbird: Enable AXP221 DC5LDO regulator as "vdd-cpus"

The DC5LDO regulator supplies VDD-CPUS, which is for the embedded
controller in the A31 SoC.

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