Introduce initial driver for PMSU on Armada38x

This is a stub for PMSU driver. Note that it cannot be used to set the
secondary core boot address during attach because drivers are attached
later than SI_SUB_CPU sysinit where cores are started.
Setting the boot address should be done manually in platform_mp_start_ap().

SMP is working fine with this commit and was enabled in Armada38x kernel
configuration file.

Obtained from: Semihalf
Sponsored by: Stormshield
Submitted by: Michal Stanek <mst@semihalf.com>
Differential revision: https://reviews.freebsd.org/D4427