tests/kernel/device: add missing `#power-domain-cells` in overlays

This commits defines the `#power-domain-cells` properties for fakedomain
nodes in the HiFive Unmatched devicetree overlay file.

Without this change, this tests fails to build for the `hifive_unmatched`
Zephyr target.

Fixes #80503.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>