gicv3: Change how we initialize its children.

The current code is written such that all the attach routines can do so
in parallel. However, newbus serializes children today, and is likely to
do so in the future. Only allocate memory for the first time. Add an
assertion that this memory is allocated for larger units.

Sponsored by: Netflix
Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D44032

(cherry picked from commit 15c8a610a80dfe1980e043174d0fcc8034868676)