MAINTAINERS: usb gadget: add Mattijs

It seems that Lukasz and Marek could get some help in maintaining the
usb gadget drivers.

Assign myself as maintainer.

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