[sival] Enable high UART baud rates on silicon

[jwnrt]

Thanks to #23379 we can now test high baud rates (1MBd, 1.5Mbd) so we enable this on silicon targets.

It's still disabled on the FPGA which cannot clock high enough to test these.

Thanks to @AlexJones0 for the help debugging.

[alees24]

This looks good, and a very welcome improvement, thanks.

[github-actions]

Backport failed for master, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin master
git worktree add -d .worktree/backport-23605-to-master origin/master
cd .worktree/backport-23605-to-master
git switch --create backport-23605-to-master
git cherry-pick -x 1c6a9a5c4af977ff676ef3f6a8f57b5e206021d0