stm32mp1: Allow to activate CONFIG_DEBUG_UART

Add the needed information to enable the debug uart
to have printf before the serial driver probe
(so before probe for clock, pincontrol and reset drivers)

To enable the debug on uart 4 (default console):
+ CONFIG_DEBUG_UART=y
+ CONFIG_DEBUG_UART_STM32=y

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