pseries: Fix incorrect initialization of interrupt controller

Currently in the reset code for the XICS interrupt controller, we
initialize the pending_priority field to 0 (most favored, by XICS
convention).  This is incorrect, since there is no pending interrupt, it
should be set to least favored - 0xff.  At the moment our XICS
implementation doesn't get hurt by this edge case, but it does confuse the
upcoming kernel XICS implementation.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
