arm64: Attach the vgic even if there is no ITS

If there is no ITS device so no memory resources the gicv3 driver would
exit attaching early. This caused the vgic driver to also not be
attached, even if it could.

Reported by: novel@
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D54791

(cherry picked from commit c12d6cc326b70326d776324067bdf07e4fa328aa)