Add initial support for SMP on Armada38x

• Add file sys/arm/mv/armada38x/armada38x_mp.c
• Set mp_maxid and mp_ncpus based on FDT unless SCU register indicates only one core
• Boot CPU1 in platform_mp_start_ap()
• IPI range defined

Obtained from: Semihalf
Sponsored by: Stormshield
Submitted by: Michal Stanek <mst@semihalf.com>
Differential revision: https://reviews.freebsd.org/D4426