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>

Documentation/devicetree/bindings/mtd/partition.txt

@@ -30,6 +30,8 @@ Optional properties:
   partition should only be mounted read-only. This is usually used for flash
   partitions containing early-boot firmware images or data which should not be
   clobbered.
+- lock : Do not unlock the partition at initialization time (not supported on
+  all devices)
 
 Examples: