MAINTAINERS: Add spi-nor device tree binding under SPI NOR maintainers

The binding is for the SPI NOR subsystem so the patches should be sent
to its maintainers. Currently the patches get sent to Rob Herring and
MTD maintainers who might not always have the full picture.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20210923191026.19184-1-p.yadav@ti.com