mtd: ofpart: document the lock flag.

The lock flag of ofpart is undocumented. Add to binding doc.

Signed-off-by: Michal Suchanek <hramrach@gmail.com>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>