ppc: Add stub implementation of TRIG SPRs

Linux sets these to control cache flush behaviour on Power9. Supervisor
and hypervisor are allowed to write, and reads are noops.

Add implementations to avoid noisy messages when booting Linux under the
pseries machine with guest_errors enabled.

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Cédric Le Goater <clg@kaod.org>