Forward-port PIRQ table updates from 2.4.x

This separates out the PIRQ table parsing to vendor-specific
code, which allows us to handle specific vendor quirks. In
particular, SiS has a really funky notion of what PCI device
ID's are meant to be.

Some hardware designers seem to be hitting the recreational
drugs a bit too heavily. Tssk, tssk.

The Sis96x irq routing update is confirmed to fix at least
one laptop.