tests: kernel: device: add missing node in hifive board overlays

A follow-up to commit d6ce2f4f41dd585a40f1d60f12536a8525b33449 that
introduced a new fakedeferdriver node in app.overlay, but due to how
overlays are applied, the board specific overlay files also need to
be updated to avoid build errors, as they override whatever app.overlay
has.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>
2 files changed