ARM: stm32: Add missing build of ST DFU virt code on DH STM32MP1 DHSOM

ARM: stm32: Add missing build of ST DFU virt code on DH STM32MP1 DHSOM

Commit 6d91f0a3a14d ("board: st: common: cleanup dfu support") split
the vendor-specific DFU implementation into two files, but failed to
update other non-ST boards. This did not lead to noticeable breakage
with plain simple dfu-util, but it makes the ST proprietary programmer
CLI tool end in an infinite loop. Update the Makefile accordingly to
allow even that kind of tooling to work.

Fixes: 6d91f0a3a14d ("board: st: common: cleanup dfu support")
Signed-off-by: Marek Vasut <marek.vasut@mailbox.org>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>