board: stm32mp1: move the function board_debug_uart_init in spl.c

Move the debug function board_debug_uart_init in spl.c
as the debug_uart_init() function is called in arch_cpu_init()
only for SPL and remove the board.c file.

For TFABOOT, the UART TX pin configuration is done in TF-A.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>