MAINTAINERS: add tree link for fastboot

Fastboot patches go through the u-boot-dfu tree.

Add a link in the maintainers file for it.

Link: https://lore.kernel.org/r/20240516-gadget-maintainer-v2-1-23cc916df434@baylibre.com


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