syzbot has bisected this bug to:

commit 2de50e9674fc4ca3c6174b04477f69eb26b4ee31
Author: Russell Currey [off-list ref]
Date:   Mon Feb 8 04:08:20 2016 +0000

     powerpc/powernv: Remove support for p5ioc2

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?xaf042ae00000
start commit:   9455d25f Merge tag 'ntb-5.5' of git://github.com/jonmason/..
git tree:       upstream
final crash:    https://syzkaller.appspot.com/x/report.txt?xaf042ae00000
console output: https://syzkaller.appspot.com/x/log.txt?xaf042ae00000
kernel config:  https://syzkaller.appspot.com/x/.config?xz3b8f5088d4043a
dashboard link: https://syzkaller.appspot.com/bug?extid™8dec6452146bd7a90c
syz repro:      https://syzkaller.appspot.com/x/repro.syz?xfa5c2ee00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?xe327f2e00000

Reported-by: syzbot+998dec6452146bd7a90c@syzkaller.appspotmail.com
Fixes: 2de50e9674fc ("powerpc/powernv: Remove support for p5ioc2")

For information about bisection process see: https://goo.gl/tpsmEJ#bisection