mtd: spi-nor: Add lock/unlock support for f25l32pa

This chip has write protection enabled on power-up,
so this flag is necessary to support write operations.

Signed-off-by: Victor Shyba <victor1984@riseup.net>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>