MFC 305546:
When synchronising the instruction and data caches we only need to clean
the data cache to the point of unification. This is the point where the
two caches are unified to a single unified cache so cleaning past here
is just extra unneeded work.

Obtained from: ABT Systems Ltd
Sponsored by: The FreeBSD Foundation

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

Fix possible coherency issues between PEs related to I-cache

Basing on B.2.3.4:
Synchronization and coherency issues between data and
instruction accesses.

To ensure that modified instructions are visible to all PEs
(Processing Elements) in a shareability domain one need to
perform following sequence:
1. Clean D-cache
2. Ensure the visibility of data cleaned from cache
3. Invalidate I-cache
4. Ensure completion
5. In SMP system PE must issue isb to ensure execution of the
modified instructions

Reviewed by: andrew
Obtained from: Semihalf
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3106

Bring in the start of the arm64 kernel.

This is only the minimum set of files needed to boot in qemu. As such it is
missing a few things.

The bus_dma code is currently only stub functions with a full implementation
from the development tree to follow.

The gic driver has been copied as the interrupt framework is different. It
is expected the two drivers will be merged by the arm intrng project,
however this will need to be imported into the tree and support for arm64
would need to be added.

This includes code developed by myself, SemiHalf, Ed Maste, and Robin
Randhawa from ARM. This has been funded by the FreeBSD Foundation, with
early development by myself in my spare time with assistance from Robin.

Differential Revision: https://reviews.freebsd.org/D2199
Reviewed by: emaste, imp
Relnotes: yes
Sponsored by: The FreeBSD Foundation