mtd: spi-nor: Enable locking for n25q512ax3/n25q512a

n25q512ax3 and n25q512a use the 4 bit Block Protection scheme.
Enable locking for both. Tested on n25q512ax3. The other is modified
following the datasheet.

Signed-off-by: Jungseung Lee <js07.lee@samsung.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>