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>

MAINTAINERS

@@ -62,6 +62,7 @@ M:	Igor Opaniuk <igor.opaniuk@gmail.com>
 M:	Mattijs Korpershoek <mkorpershoek@baylibre.com>
 R:	Sam Protsenko <semen.protsenko@linaro.org>
 S:	Maintained
+T:	git https://source.denx.de/u-boot/custodians/u-boot-dfu.git
 F:	boot/android_ab.c
 F:	cmd/ab_select.c
 F:	doc/android/ab.rst
@@ -72,6 +73,7 @@ ANDROID AVB
 M:	Igor Opaniuk <igor.opaniuk@gmail.com>
 M:	Mattijs Korpershoek <mkorpershoek@baylibre.com>
 S:	Maintained
+T:	git https://source.denx.de/u-boot/custodians/u-boot-dfu.git
 F:	cmd/avb.c
 F:	common/avb_verify.c
 F:	doc/android/avb2.rst