This takes the SPCR code currently in uart_cpu_arm64.c, moves it into a new uart_cpu_acpi.c, and uses it from both arm64 and x86. (In the process there is some refactoring and I added some comments to help me understand the code.)
Which spec are you looking at?
says "0 = as is, operating system relies on the current configuration of serial port until the full featured driver will be initialized."
I'll add the comment back anyway...