MAINTAINERS: Add an entry for SPI NOR

Add myself as co-maintainer for U-Boot SPI NOR subsystem.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
[jagan: drop mtd/spi file from SPI entry]
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>