Allow setting access-width for UART registers.

Allow setting access-width for UART registers.

This is required for FDT's standard "reg-io-width" property
(similar to "reg-shift" property) found in many DTS files.

This fixes operation on Altera Arria 10 SOC Development Kit,
where standard ns8250 uart allows 4-byte access only.

Reviewed by:	kan, marcel
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D9785