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 <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Tested-by: Dmitry Osipenko <[email protected]> # crosvm
Link: https://lore.kernel.org/r/[email protected]

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