MAINTAINERS: Add entry for SPI NAND framework and drivers

MAINTAINERS: Add entry for SPI NAND framework and drivers

In [1] Michael agreed on taking patches for SPI NAND through the RAW
NAND tree. Add a dedicated entry to the MAINTAINERS file which adds
Michael and Dario as maintainers and myself as reviewer.

[1] https://lists.denx.de/pipermail/u-boot/2023-February/508571.html

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Cc: Michael Nazzareno Trimarchi <michael@amarulasolutions.com>
Cc: Tom Rini <trini@konsulko.com>
Link: https://lore.kernel.org/all/20230213094626.50957-1-frieder@fris.de/
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>