[spcr] Accept alternative type value for a 16550-compatible UART

[spcr] Accept alternative type value for a 16550-compatible UART

Some systems (observed on an AWS EC2 m7i.metal-24xl instance in
eu-south-2) use the newer "16550-compatible with parameters defined in
Generic Address Structure" type value.  (There does not appear to be
any particular reason why the newer value needs to be used: the UART
is still a standard 16550 with single-byte registers.)

Accept this additional type value for a 16550-compatible UART.

Signed-off-by: Michael Brown <mcb30@ipxe.org>