KVM: PPC: reserve a capability number for multitce support

This is to reserve a capablity number for upcoming support
of H_PUT_TCE_INDIRECT and H_STUFF_TCE pseries hypercalls
which support mulptiple DMA map/unmap operations per one call.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Gleb Natapov <gleb@redhat.com>