Update devmap to work with arm64

[andrew]

Summary

We can reuse most of devmap on arm64. This updates it to handle this. We
reserve an L2 page (2MiB with 4k granules) for early devices, e.g. the UART.
It also updates the code to handle the different pmap implementations and
memory layout.

[imp]

sys/arm/arm/devmap.c

31–33

I'd update this comment to say this is shared between aarch64 and arm because of the strong expectation that arm/arm/foo is private to arm.

[andrew]

Add a comment on this file being shared

[andrew]

https://svnweb.freebsd.org/changeset/base/281493