Enable SCU unit for Armada38x

Valid SCU operation is necessary for SMP interoperability.
Initialization function armada38x_enable_scu() was added.

Reviewed by: andrew, ian
Obtained from: Semihalf
Sponsored by: Stormshield
Submitted by: Bartosz Szczepanek <bsz@semihalf.com>
Differential revision: https://reviews.freebsd.org/D4220