mtd: spi-nor: sst: Use flash late_init() for locking

Locking is not described in JESD216 SFDP standard, place the
locking init in late_init().

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20211029172633.886453-8-tudor.ambarus@microchip.com