target/ppc: add support for hypervisor doorbells on book3s CPUs

target/ppc: add support for hypervisor doorbells on book3s CPUs

The hypervisor doorbells are used by skiboot and Linux on POWER9
processors to wake up secondaries.

This adds processor control support to the Server architecture by
reusing the Embedded support. They are very similar, only the bits
definition of the CPU identifier differ.

Still to be done is message broadcast to all threads of the same
processor.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>