ARM: sun7i: a20: cubieboard2: Enable AXP209's ACIN power supply

The ACIN pins of the AXP209 PMIC on the Cubieboard 2 are tied to the
DC jack on the board.

Enable the ACIN power supply.

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