ARM: stm32: Add missing build of debug UART init code on DH STM32MP1 DHSOM

ARM: stm32: Add missing build of debug UART init code on DH STM32MP1 DHSOM

Commit c37a6684818d ("stm32mp: fix compilation issue with DEBUG_UART")
split the debug UART initialization code into two files, but failed to
update other non-ST boards. This did not lead to noticeable breakage
until debug UART is enabled, which is not the default. Update the
Makefile accordingly to allow debug UART to work.

Fixes: c37a6684818d ("stm32mp: fix compilation issue with DEBUG_UART")
Signed-off-by: Marek Vasut <marek.vasut@mailbox.org>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>