reset: ath79: add driver Kconfig option

Visible only if COMPILE_TEST is enabled, this allows to include the
driver in build tests.

Acked-by: Aban Bedel <albeu@free.fr>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>