powerpc/shared: Use static key to detect shared processor

With the static key shared processor available, is_shared_processor()
can return without having to query the lppaca structure.

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Acked-by: Phil Auld <pauld@redhat.com>
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191213035036.6913-2-mpe@ellerman.id.au