x86: acpi: Add SPCR table description

Add SPCR table description as it provided in Linux kernel.

Port subtype for ACPI_DBG2_SERIAL_PORT is used as an interface type in SPCR.
Thus, provide a set of definitions to be utilized later.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>