MFC r306263, r306268

r306263 by andrew:
Move cpu_reset to be a platform method to allow multiple implementations.

Reviewed by: mmel
Sponsored by: ABT Systems Ltd
Differential Revision: https://reviews.freebsd.org/D8010

r306268 by andrew:
Also implement platform_cpu_reset on bcm2836

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add SMP support to the ARM PLATFORM code. This will allow us to have
different methods to start the secondary cores in a kernel built for
multiple SoCs, e.g. with the Allwinner A20 and A31.

Sponsored by: ABT systems Ltd
Differential Revision: https://reviews.freebsd.org/D5466

Add the start of the ARM platform code. This is based on the PowerPC
platform code, it is expected these will be merged in the future when the
ARM code is more complete.

Until more boards can be tested only use this with the Raspberry Pi and
rrename the functions on the other SoCs.

Reviewed by: ian@