allwinner: Scan AXP803 FDT node to setup initial power rails

allwinner: Scan AXP803 FDT node to setup initial power rails

Now that we have a pointer to the device tree blob, let's use that to
do some initial setup of the PMIC:
- We scan the DT for the compatible string to find the PMIC node.
- We switch the N_VBUSEN pin if the DT property tells us so.
- We scan over all regulator subnodes, and switch DC1SW if there is at
least one other node referencing it (judging by the existence of a
phandle property in that subnode).
This is just the first part of the setup, a follow up patch will setup
voltages.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>