Backport of r338074 - generalize uart_bus_probe and add SNPS support to x86

Submitted by: Rajesh Kumar
Differential Revision: https://reviews.freebsd.org/D17381

Revert r330897:

This was intended to be a non-functional change. It wasn't. The commit
message was thus wrong. In addition it broke arm, and merged crypto
related code.

Revert with prejudice.

This revert skips files touched in r316370 since that commit was since
MFCed. This revert also skips files that require $FreeBSD$ property
changes.

Thank you to those who helped me get out of this mess including but not
limited to gonzo, kevans, rgrimes.

Requested by: gjb (re)

Partial merge of the SPDX changes

These changes are incomplete but are making it difficult
to determine what other changes can/should be merged.

No objections from: pfg

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

Merge from projects/mips to head by hand:

Merge support for very early alchemy port. I wouldn't merge this
except I don't want it to get lost when we retire projects/mips.
Should be consiered pre-alpha at this stage. Also, alchemy is now
owned by rmi, but started out life as a separate processor line, so
I'm leaving it in its own directory rather than try to shoe-horn it
into the unrelated rmi directory. Its future location is an open
question.

- Revert changes accidentally killed by merge operation

First milestone on Alchemy port: Simplest kernel now compiles.