MIPS: Netlogic: Avoid using fixed PIC IRT index

The index for a device interrupt in the PIC interrupt routing table
changes for different chips in the XLP family.  Avoid using the fixed
entries and derive the index value from the SoC device header.

Add workarounds for some devices which do not report the IRT index
correctly.

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Patchwork: http://patchwork.linux-mips.org/patch/5025/
Acked-by: John Crispin <blogic@openwrt.org>