allwinner: Add platform PSCI functions required for SMP

The reset vector entry point is preserved across CPU resets, so it only
needs to be set once at boot.

Hotplugged CPUs are not actually powered down, but are put in a wfi with
the GIC disconnected.

With this commit, Linux is able to enable, hotplug and use all four CPUs.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>