MAINTAINERS: Add Dario and Michael as NAND maintainers

Both of us are working on NAND subsystem on several architectures and
we have boards and projects to improve the subsystem in uboot. The idea
is to guarantee quick feedback on patches sent on mailing list and most
of the time the possibilities to test them.

Cc: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Acked-by: Tom Rini <trini@konsulko.com>