KVM: PPC: Add dummy kvm_arch_init_irq_routing()

KVM: PPC: Add dummy kvm_arch_init_irq_routing()

The common KVM code insists on calling kvm_arch_init_irq_routing()
as soon as it sees kernel header support for it (regardless of whether
QEMU supports it).  Provide a dummy function to satisfy this.

Unlike x86, PPC does not have one default irqchip, so there's no common
code that we'd stick here.  Even if you ignore the routes themselves,
which even on x86 are not set up in this function, the initial XICS
kernel implementation will not support IRQ routing, so it's best to
leave even the general feature flags up to the specific irqchip code.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>