MAINTAINERS: add custodian tree info for AVB/AB

MAINTAINERS: add custodian tree info for AVB/AB

Add information about a custodian tree [1] for AVB/AB, which is
maintained by Mattijs Korpershoek.

[1] https://source.denx.de/u-boot/custodians/u-boot-dfu

Signed-off-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20240213073638.1125429-1-igor.opaniuk@foundries.io
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>