power: pmic: sunxi: add AXP313 SPL driver

On boards using the AXP313 PMIC, the DRAM rail is often not setup
correctly at reset time, so we have to program the PMIC very early in
the SPL, before running the DRAM initialisation.

Add a simple AXP313 PMIC driver that knows about DCDC2(CPU) and
DCDC3(DRAM), so that we can bump up the voltage before the DRAM init.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>