MAINTAINERS: Add Michael and Pratyush as designated reviewers for SPI NOR

It's already been the case for some time that Michael and Pratyush
are reviewing SPI NOR patches. Update MAINTAINERS to reflect reality.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Acked-by: Michael Walle <michael@walle.cc>
Acked-by: Pratyush Yadav <p.yadav@ti.com>
Acked-by: Vignesh Raghavendra <vigneshr@ti.com>
Link: https://lore.kernel.org/r/20210308092333.80521-2-tudor.ambarus@microchip.com