pirq_routing: Allow routing with more than 4 PIRQ links

pirq_routing_irqs assumed that only four links are available for PIRQ
routing, INTA to INTD. Some chipsets provide more, up to INTH.
When pirq_routing_irqs found a link number greater than 4 in the pirq table,
it would not assign that IRQ. This is a shame, as it limits the flexibility
of routing IRQs.
Make the maximum number of links a Kconfig variable, and modify the code to
respect it. This works beatifully on the VX900, which provides 8 routable
While we're at it, also refactor pirq_routing_irqs, and add some much
needed comments.
Rename pirq_routing_irqs to pirq_route_irqs to demistify the role of this
The copyrights added were determined from git log filename.

Change-Id: I4b565315404c65b871406f616474e2cc9e6e013e
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-on: http://review.coreboot.org/1482
Tested-by: build bot (Jenkins)
3 files changed