uart: allow UART_DEV_DBGPORT for fdt consoles

Allow fdt devices to be used as debug ports for gdb(4).

A debug console can be specified with the "freebsd,debug-path" property
in the device tree's /chosen node, or using the environment variable
hw.fdt.dbgport.

The device should be specified by its name in the device tree, for
example hw.fdt.dbgport="serial2".

PR: 251053
Submitted by: Dmitry Salychev <dsl@mcusim.org>
Submitted by: stevek (original patch, D5986)
Reviewed by: andrew, mhorne
Differential Revision: https://reviews.freebsd.org/D27422