MAINTAINERS: Add maintainers for DHCOM i.MX6 and DHCOM/DHCOR STM32MP1

Add maintainers for DH electronics DHCOM i.MX6
and DHCOM/DHCOR STM32MP1 boards.

Signed-off-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: kernel@dh-electronics.com
Cc: arnd@arndb.de
Link: https://lore.kernel.org/r/20211025073706.2794-1-cniedermaier@dh-electronics.com'
To: soc@kernel.org
To: linux-kernel@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>