mtd: spi-nor: Add entries for mt25q variants

Add entries for mt25q*256a and mt25q*512a flashes. These are similar to
existing n25q variants but support stateless 4 byte addressing opcodes

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>