The Microsoft Debug Port Table 2 (DBG2) specification revision
May 31, 2017 added definition of the 16550-compatible Serial Port
Subtype with parameters defined in Generic Address Structure (GAS) [1]

Add its support in the SPCR table parsing routine.

[1] https://docs.microsoft.com/en-us/windows-hardware/drivers/bringup/acpi-debug-port-tableSigned-off-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>