ppce500_mpc8544ds: Fix compile with --enable-debug and --disable-kvm

When configured with --enable-debug, we compile without optimization.
This means that the function mpc8544_copy_soc_cell() in ppce500_mpc8544ds.c
is not optimized out, even though it is never called without kvm.  That in
turn causes a link failure, because it calls the function
kvmppc_read_host_property() which is in kvm_ppc.o and therefore not
included in a --disable-kvm build.

This patch fixes the problem by providing a dummy stub for
kvmppc_read_host_property() in kvm_ppc.h when !CONFIG_KVM.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>