MAINTAINERS: Add entry for ULTRATRONIK BOARD SUPPORT

MAINTAINERS: Add entry for ULTRATRONIK BOARD SUPPORT

Add a new MAINTAINERS section for the ULTRATRONIK BOARD SUPPORT, covering
the stm32mp157c-ultra-fly-sbc.dts board support file.

This ensures that maintainers of this board are properly listed and can be
notified for any relevant changes.

Signed-off-by: Goran Rađenović <goran.radni@gmail.com>
Signed-off-by: Börge Strümpfel <boerge.struempfel@gmail.com>
Link: https://lore.kernel.org/r/20250508143818.2574558-4-goran.radni@gmail.com
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>