PowerNV: set LPCR[LPES] correctly

Make sure to set LPCR[LPES] so that external interrupts set SRR0 and SRR1
instead of HSRR0 and HSRR1. Without this, external interrupt handlers would
get the wrong MSR value when executing, causing eventual madness.

Created by: Nathan Whitehorn <nwhitehorn@freebsd.org>
Submitted by: Wojciech Macek <wma@freebsd.org>
Sponsored by: FreeBSD Foundation