mtd: ofpart: Replicate mtd cmdline "lk" option with device tree "lock" property

The mtd partition command line parser already supports a "lk" option to mask
MTD_POWERUP_LOCK. This extends that same functionality to device tree
partition specifications.

Signed-off-by: Josh Radel <jradel@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>