cns3xxx: support isolated PCI interrupts on newer Laguna PCBs

cns3xxx: support isolated PCI interrupts on newer Laguna PCBs

The cns3xxx uses irq61 for pcie0_intr which in the case of a PCIe-to-PCI
bridge ends up combining INTA/B/C/D on a single ARM CPU interrupt. This is
not optimal when you have multiple cores. To overcome this limitation an
enhancement was made on newer Laguna PCB's that support miniPCI cards
to route the INTA/B/C/D signals to unique external ARM CPU interrupts which
can help balance CPU core utilization and in some cases increase overall
system performance or responsiveness.

For more details see:
 http://trac.gateworks.com/wiki/multicoreprocessing#PCIInterruptsteering

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Backport of r42400

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42402 3c298f89-4303-0410-b956-a3cf2f4a3e73