mtd: spi-nor: enable locking support for MX25L12805D

Macronix MX25L12805D supports locking with 4 block
protection bits in its status register. Add the corresponding
flag in order to clear these bits when unloking the flash.

Otherwise, the flash might not be writable depending on the state
left by the bootloader.

Tested-on: Ubiquiti UniFi AC Lite (ath79)

Signed-off-by: David Bauer <mail@david-bauer.net>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Michael Walle <michael@walle.cc>