arm: prune imx5 support from the tree

The IMX5 configs were removed in advance of FreeBSD 14.0 in
cdb0c2a73df ("arm: Remove IMX5 specific kernel configs"). This code
isn't built with GENERIC and doesn't actually build today as-is, so
let's remove it to avoid needless maintenance work to it that won't be
tested. As usual, revival is welcome with a committed user and work to
maintain it with upstream DTS and, ideally, in GENERIC.

I note that vt_early_fb is now effectively orphaned as nothing else will
use it, but I haven't yet removed it since I have not done anything to
ascertain if it could be integrated easily enough for other SoC. It is
among the files that doesn't actually build with today's clang, though.

Reviewed by: imp, manu
Differential Revision: https://reviews.freebsd.org/D41836