ppc/xive: Provide silent escalation support

ppc/xive: Provide silent escalation support

When the 's' bit is set the escalation is said to be 'silent' or
'silent/gather'. In such configuration, the notification sequence is
skipped and only the escalation sequence is performed. This is used to
configure all the EQs of a vCPU to escalate on a single EQ which will
then target the hypervisor.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20190718115420.19919-8-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>