MAINTAINERS: add USB gadget regex to u-boot-dfu tree

We try to split work with Marek on USB as following:
- Mattijs handles USB gadget
- Marek handles the rest of USB

Add additional gadget patterns to the maintainers file so that I
get cc'ed more often on USB gadget patches.

Acked-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20240516-gadget-maintainer-v2-2-23cc916df434@baylibre.com


Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>