uart: Add UART_ACPI_SPCR_CLASS

We don't care about most of acpi_uart_compat_data in the spcr code.
Split out the mapping from the SPCR interface type to uart class into
a new struct. This allows new SPCR interface types to be added that
don't have an entry in the DSDT, e.g. some PCI uarts may not.

Reviewed by: imp, jhibbits
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D51878