Add support for SMP. This uses the FDT data to find the CPUs to start on,
and psci to start them. I expect ACPI support to be added later.

This has been tested on qemu with 2 cpus as that is the current value of
MAXCPUS. This is expected to be increased in the future as FreeBSD has
been tested on 48 cores on the Cavium ThunderX hardware.

Partially based on a patch from Robin Randhawa from ARM.

Approved by: ABT Systems Ltd
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3024