MAINTAINERS: fastboot: add Mattijs

Fastboot has been marked as orphaned since 2021.
Since I'm interested in maintaining this, assign myself.

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Lukasz Majewski <lukma@denx.de>