arm64: Check for intrng-reported errors in gicv3_its

Currently, any errors when adding a PIC child handler are ignored,
instead just continuing on to registering that PIC as an MSI, and
ignoring any errors that occur for that too.

Reviewed by: andrew
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D33342

(cherry picked from commit 3582b9e372c2556adc0fcc42466ead8b2bda7850)