ARM: stm32: Enable DFU MTD support on DHSOM

All the STM32MP1 based DHSOM have SPI NOR from which they boot,
enable DFU_MTD support to make it possible to expose that SPI NOR
via the DFU.

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