mtd: sf: Add support of sst26wf* flash ICs protection ops

sst26wf flash series block protection implementation differs
from other SST series, so add specific implementation
flash_lock/flash_unlock/flash_is_locked functions for sst26wf
flash ICs.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>