dev/ofw: Add interrupt-map support

Follow interrupt-map properties until either an interrupt controller or
invalid node is found. In the former case return the translated details
for the interrupt controller driver to decode. In the latter case return
0 as an error.

Tested on the Arm DTS files that use interrupt-maps and with a
userspace test.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D51257