target/ppc: Add Directed Privileged Door-bell Exception State (DPDES) SPR

target/ppc: Add Directed Privileged Door-bell Exception State (DPDES) SPR

DPDES stores a status of a doorbell message and if it is lost in
migration, the destination CPU won't receive it. This does not hit us
much as IPIs complete too quick to catch a pending one and even if
we missed one, broadcasts happen often enough to wake that CPU.

This defines DPDES and registers with KVM for migration.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-Id: <20190816061733.53572-1-aik@ozlabs.ru>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>