powerpc/corenet: use the mixed mode of MPIC when enabling CPU hotplug

Core reset may cause issue if using the proxy mode of MPIC.
Use the mixed mode of MPIC if enabling CPU hotplug.

Signed-off-by: Chenhui Zhao <chenhui.zhao@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>