uart: Ingore pl011 historic mistakes

Some veresions of EDK-II and QEMU reported the wrong values for the
register shift and the region I/O size. Detect those and set it to the
correct values. In general, anything other than a shift of 2 and a
regio width of 4 (bytes, or 32 bits) is a mistake. However, allow
for overrides in the future by only overriding the buggy values.
Otherwise, we will fail to boot.

PR: 282936
Sponsored by: Netflix
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D47946