x86/PCI: Add $IRT PIRQ routing table support

x86/PCI: Add $IRT PIRQ routing table support

Handle the $IRT PCI IRQ Routing Table format used by AMI for its BCP
(BIOS Configuration Program) external tool meant for tweaking BIOS
structures without the need to rebuild it from sources[1].

The $IRT format has been invented by AMI before Microsoft has come up
with its $PIR format and a $IRT table is therefore there in some systems
that lack a $PIR table, such as the DataExpert EXP8449 mainboard based
on the ALi FinALi 486 chipset (M1489/M1487), which predates DMI 2.0 and
cannot therefore be easily identified at run time.

Unlike with the $PIR format there is no alignment guarantee as to the
placement of the $IRT table, so scan the whole BIOS area bytewise.

Credit to Michal Necasek for helping me chase documentation for the
format.

References:

[1] "What is BCP? - AMI", <https://www.ami.com/what-is-bcp/>

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com> # crosvm
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2203302228410.9038@angie.orcam.me.uk