ARM: stm32: Update dhelectronics/dh_stm32mp1/MAINTAINERS file

Adding new DH electronics mailing list.
Signed-off-by: NChristoph Niedermaier <cniedermaier@dh-electronics.com>
Reviewed-by: NPatrick Delaunay <patrick.delaunay@foss.st.com>

board/dhelectronics/dh_stm32mp1/MAINTAINERS

 DH_STM32MP1_PDK2 BOARD
 M:	Marek Vasut <marex@denx.de>
+L:	u-boot@dh-electronics.com
 S:	Maintained
 F:	arch/arm/dts/stm32mp15xx-dhcom*
 F:	board/dhelectronics/dh_stm32mp1/