mtd: spi-nor: add unlock all config option

mtd: spi-nor: add unlock all config option

Provide an explicit configuration option to disable default "unlock all"
of any flash chip which supports locking. It doesn't make sense to
automatically unprotect the entire flash on each u-boot startup if the
block protection bits are actually used.

Traditionally, the unlock was there to be able to write to flash devices
which powered-up with the block protection bits set. Over time this
feature creeped into all flash devices which support locking.

For a more detailed description and discussion see:
https://lore.kernel.org/linux-mtd/20201203162959.29589-8-michael@walle.cc/

Keep things simple in u-boot and just provide a configration option to
disable this behavior which can be set per board.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>