uart: Add ns8250 ACPI entry for SPCR rev 2

Summary:
SPCR rev 2 adds a series of new device types, with 0x12 being
"16550-compatible with parameters defined in the Generic Address
Structure". Since we look for the parameters already in the GAS there's
nothing extra for us to do beyond adding the entry so that it probes
successfully.

Reviewed by: imp, andrew
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D51771