C6X: remove megamod-pic requirement on direct-mapped core pic

The megamodule PIC cascades a number of interrupt sources into the core
priority PIC. The megamodule code depends on the core hardware interrupt
numbers being mapped one-to-one with regard to linux interrupt numbers.
This patch removes that dependence in order to pave the way for removing
the direct mapping in the core PIC code.

Signed-off-by: Mark Salter <msalter@redhat.com>