Add a driver for the ARM Power State Coordination Interface (PSCI). This
handles versions 0.1 and 0.2 of the standard on 32-bit ARM.

With this driver we can shutdown in QEMU. Further work is needed to
turn secondary cores on on boot and to support later revisions of the
specification.

Submitted by: Robin Randhawa <Robin.Randhawa at ARM.com>
Sponsored by: The FreeBSD Foundation