x86/PCI: Add support for the Intel 82426EX PIRQ router

x86/PCI: Add support for the Intel 82426EX PIRQ router

The Intel 82426EX ISA Bridge (IB), a part of the Intel 82420EX PCIset,
implements PCI interrupt steering with a PIRQ router in the form of two
PIRQ Route Control registers, available in the PCI configuration space
at locations 0x66 and 0x67 for the PIRQ0# and PIRQ1# lines respectively.

The semantics is the same as with the PIIX router, however it is not
clear if BIOSes use register indices or line numbers as the cookie to
identify PCI interrupts in their routing tables and therefore support
either scheme.

The IB is directly attached to the Intel 82425EX PCI System Controller
(PSC) component of the chipset via a dedicated PSC/IB Link interface
rather than the host bus or PCI.  Therefore it does not itself appear in
the PCI configuration space even though it responds to configuration
cycles addressing registers it implements.  Use 82425EX's identification
then for determining the presence of the IB.

References:

[1] "82420EX PCIset Data Sheet, 82425EX PCI System Controller (PSC) and
    82426EX ISA Bridge (IB)", Intel Corporation, Order Number:
    290488-004, December 1995, Section 3.3.18 "PIRQ1RC/PIRQ0RC--PIRQ
    Route Control Registers", p. 61

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2107200213490.9461@angie.orcam.me.uk