KVM: powerpc: Use Makefile.kvm for common files

It's all fairly baroque but in the end, I don't think there's any reason
for $(KVM)/irqchip.o to have been handled differently, as they all end
up in $(kvm-y) in the end anyway, regardless of whether they get there
via $(common-objs-y) and the CPU-specific object lists.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Message-Id: <20211121125451.9489-7-dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>