mtd: spi-nor: Introduce spi_nor_set_addr_width()

Parsing of flash parameters were interleaved with setting of the
nor addr width. Dedicate a function for setting nor addr width.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Vignesh Raghavendra <vigneshr@ti.com>