Add SMP support for the Allwinner A31 and A31s. This updated the existing
code for the A20 to use the new PLATFORM_SMP interface, and extends it to
add support for the new SoCs allowing for both to coexist within the same
kernel.

Submitted by: Emmanuel Vadot <manu@bidouilliste.com>
Reviewed by: jmcneill
Differential Revision: https://reviews.freebsd.org/D5342