of: overlay: Fix function name disparity

Fixes the following W=1 kernel build warning(s):

 drivers/of/overlay.c:147: warning: expecting prototype for of_overlay_notifier_register(). Prototype was for of_overlay_notifier_unregister() instead

Cc: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210318104036.3175910-10-lee.jones@linaro.org