powerpc/virtex: refactor intc driver and add support for i8259 cascading

This patch refactors some of the xilinx_intc interrupt controller driver
and adds support for cascading an i8259 off one of the irq lines.

This patch was based on the ML510 support work done by Roderick
Colenbrander.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>