reset: Add of_reset_control_get to reset.h

of_reset_control_get is not declared static in drivers/reset/core.c, which
is correct as we want to use it elsewhere too. But it does not have a
protype declared anywhere under include/linux. Add a prototype / stub for it
to linux/reset.h to fix this.

Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>