term/ns8250: Use ACPI SPCR table when available to configure serial

term/ns8250: Use ACPI SPCR table when available to configure serial

"serial auto" is now equivalent to just "serial" and will use the
SPCR to discover the port if present, otherwise defaults to "com0"
as before.

This allows to support MMIO ports specified by ACPI which is needed
on AWS EC2 "metal" instances, and will enable GRUB to pickup the
port configuration specified by ACPI in other cases.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>