mtd: spi-nor: Create a ->set_4byte() method

The procedure used to enable 4 byte addressing mode depends on the NOR
device, so let's provide a hook so that manufacturer specific handling
can be implemented in a sane way.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
[tudor.ambarus@microchip.com: use nor->params.set_4byte() instead of
nor->set_4byte()]
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Vignesh Raghavendra <vigneshr@ti.com>