ARM: sun8i: a83t: cubietruck-plus: Enable PMIC part of AXP818 PMIC

The AXP813/AXP818 PMICs used with the A83T/H8 SoCs are actually 2 dies
in one package sharing the serial bus (I2C/RSB) pins. One die is the
actual PMIC. The other is an AC100 codec / RTC combo chip.

This patch enables the RSB controller and adds a device node for the
PMIC die to the Cubietruck Plus device tree. Since the AXP813 and
AXP818 are virtually identical, this patch uses the compatible string
for the former as a fallback.

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