ARM: dts: sun7i: Use axp209.dtsi on A20-OLinuXino-Micro

This commit makes use of the axp209.dtsi file to define the
AXP209 PMIC. While here, define the rails that are enabled on
this board.

Tested checking the regulator voltage varies according to the
CPU frequency.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>