stm32mp: fix compilation issue with DEBUG_UART

Fix the compilation issue when CONFIG_DEBUG_UART is activated

 drivers/serial/serial_stm32.o: in function `debug_uart_init':
 drivers/serial/serial_stm32.c:291: undefined reference to \
    `board_debug_uart_init'

The board_debug_uart_init is needed for SPL boot, called in
cpu.c::mach_cpu_init(); it is defined in board/st/stm32mp1/spl.c.

But with the removal #ifdefs patch, the function debug_uart_init() is
always compiled even if not present in the final U-Boot image.

This patch adds a file to provided this function when DEBUG_UART and SPL
are activated.

Fixes: c8b2eef5 ("stm32mp15: tidy up #ifdefs in cpu.c")
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>