arch-arm: Add recursion for DTB entry generation inside ArmISA

arch-arm: Add recursion for DTB entry generation inside ArmISA

In order to generate the ArmPMU's DTB entry, we have to enable recursion
from the ArmISA.

This commit follows this mailing list entry:
https://www.mail-archive.com/gem5-users@gem5.org/msg18401.html

Change-Id: I73012755f0f8c8d4d17278793cf16cb1e8b011df
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35555
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>