powerpc/virtex: Fix duplicate level irq events.

The interrupt controller was not handling level interrupts correctly
such that duplicate interrupts were happening. This fixes the problem
and adds edge type interrupts which are needed in Xilinx hardware.

Signed-off-by: John Linn <john.linn@xilinx.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>