ppc/xive: Make the PIPR register readonly

ppc/xive: Make the PIPR register readonly

When the hypervisor (KVM) dispatches a vCPU on a HW thread, it restores
its thread interrupt context. The Pending Interrupt Priority Register
(PIPR) is computed from the Interrupt Pending Buffer (IPB) and stores
should not be allowed to change its value.

Fixes: 207d9fe98510 ("ppc/xive: introduce the XIVE interrupt thread context")
Signed-off-by: Cédric Le Goater <[email protected]>
Message-Id: <20190630204601[email protected]>
Signed-off-by: David Gibson <[email protected]>