Allwinner produces reference board designs, which apparently most board
vendors copy from. So every H5 board I checked uses regulators which are
controlled by the same PortL GPIO pins to power the ARM CPU cores, the
DRAM and the I/O ports.
Add a SoC specific power down routine, which turns those regulators off
when ATF detects running on an H5 SoC and the rich OS triggers a
SYSTEM_POWEROFF PSCI call.

NOTE: It sounds very tempting to turn the CPU power off, but this is not
working as expected, instead the system is rebooting. Most probably this
is due to VCC-SYS also being controlled by the same GPIO line, and
turning this off requires an elaborate and not fully understood setup.
Apparently not even Allwinner reference code is turning this regulator
off. So for now we refrain to pulling down PL8, the power consumption is
quite low anyway, so we are as close to poweroff as reasonably possible.
Many thanks to Samuel for doing some research on that topic.

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