ARM: dts: stm32: Deduplicate CONFIG_OF_SPL_REMOVE_PROPS on DH STM32MP15xx DHSOM

The content of CONFIG_OF_SPL_REMOVE_PROPS is the same in both
STM32MP15xx DHCOM and DHCOR defconfigs, deduplicate the content
into stm32mp15_dhsom.config .

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>