dev/uart: Support 8-byte register access

While we only support 4-byte registers in the uart code the physical
access may be to an 8-byte register. Support this as an option on
non-i386. On i386 we lack the needed 8-byte bus_space functions.

ACPI has an option for 8-byte register io width, and FDT can be given
any size. Support these sizes, even if we don't expect to see hardware
with an 8-byte io width.

Reviewed by: imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43374