irq_domain/c6x: Convert c6x to use generic irq_domain support.

The C6X IRQ support was copied almost verbatim from the PowerPC virtual IRQ
code. The PowerPC code was used as the basis for generic irq_domain support,
so this patch mostly copies what what done to arch/powerpc by Grant Likely
in his irq_domain patch series.

Signed-off-by: Mark Salter <msalter@redhat.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Aurelien Jacquiot <a-jacquiot@ti.com>
Cc: Thomas Gleixner <tglx@linutronix.de>