powerpc/85xx: issue 15 EOI after core reset for FSL CoreNet devices

This is listed as a requirement for Freescale CoreNet based devices  (e.g
p4080ds with MPIC v4.x) after issuing a core reset to properly clear pending
interrupts.

Signed-off-by: Matthew McClintock <msm@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>