reset: Move reset-simple header out of drivers/reset

The reset-simple code can be useful for drivers outside of drivers/reset
that have a few reset controls as part of their features. Let's move it to
include/linux/reset.

Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>