reset: ath79: use devm_reset_controller_register()

Use devm_reset_controller_register() for the reset controller
registration and remove the unregister call from the .remove callback.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>