uart: Split out initilisation of the acpi devinfo

Split out the common parts of building the uart devinfo from ACPI
tables from the SPCR parser. This will be used when we support the DBG2
table to find the debug uart to be used by the kernel gdb stub.

Reviewed by: imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D44357

(cherry picked from commit 473c0b44ae8c51b2aebc51887714b2ed14de50bf)