x86: acpi: Generate SPCR table

Microsoft specifies a SPCR (Serial Port Console Redirection Table) [1].
Let's provide it in U-Boot.

[1]: https://docs.microsoft.com/en-us/windows-hardware/drivers/serports/serial-port-console-redirection-table



Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>