ARM: stm32: Deduplicate DH STM32MP15xx DHSOM defconfigs

ARM: stm32: Deduplicate DH STM32MP15xx DHSOM defconfigs

Deduplicate defconfigs for all DH STM32MP15xx DHSOM systems by factoring
out the common parts into generic stm32mp15_dhsom.config and including
those using the #include <configs/...> preprocessor macro in the current
set of board specific defconfigs. The preprocessor macro is applicable
to defconfigs as well.

This introduces no functional change, the resulting .config is identical
for all DH STM32MP15xx DHSOM systems.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>