reset: fix reset_control_lookup kerneldoc comment

Add a missing colon to fix a documentation build warning:

  ./include/linux/reset-controller.h:45: warning: Function parameter or member 'con_id' not described in 'reset_control_lookup'

Fixes: 6691dffa ("reset: add support for non-DT systems")
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>