Since current_cpu is CPUState it no longer depends on CPUPPCState.

Move ppce500_set_mpic_proxy() to a new hw/ppc/ppc_e500.h because
hw/ppc/ppc.h is too heavily using CPUPPCState and PowerPCCPU.
Signed-off-by: NAndreas Färber <afaerber@suse.de>