uart: Support EARLY_PRINTF on x86 for port-mapped COM ports

Support early printf for the ns8250 uart driver. Adding
options UART_NS8250_EARLY_PORT=0xYYY
options EARLY_PRINTF
to your kernel config will enable it. The code is rather simple minded,
so caveat emptor. This will enable printf before cninit. cninit
automatically disables this and switches to the real routine. It only
works for port-mapped COM ports, and only if you know the port's address
at compile time. It's intended for be a debugging aide, not a general
purpose thing.

Sponsored by: Netflix
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D42306