allwinner: implement system power down on H6 w/ AXP805

The AXP805 PMIC used with H6 is capable of shutting down the system.

Add support for using it to shut down the system power.

The original placeholder power off code is moved to A64 code, as it's
still TODO to implement PMIC operations for A64.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>