power: pmic: sunxi: add AXP717 SPL support

power: pmic: sunxi: add AXP717 SPL support

On boards using the AXP717 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.

Using the new generic AXP SPL driver, add the Kconfig options and
platform bits needed to support an AXP717 PMIC chip in I2C mode.
This allows to set up the correct voltage for the DRAM chips and the
CPU cores.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Ryan Walklin <ryan@testtoast.com>