Enable proper parsing of nested simlpe-buses on Marvell platforms

OF_finddevice doesn't find the "simple-bus" node, which is problematic
for Marvell platforms, using nested buses in Device Tree, like
Armada 38x SoC.

Submitted by: Arnaud Ysmal <arnaud.ysmal@stormshield.eu>
Obtained from: Stormshield
Sponsored by: Stormshield
Reviewed by: loos
Differential revision: https://reviews.freebsd.org/D10719