ACPI: SPCR: Add support for NVIDIA 16550-compatible port subtype

Add support for the NVIDIA specific 16550 subtype to SPCR table parsing
routine.

Signed-off-by: Jeff Brasen <jbrasen@nvidia.com>
Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>