ARM: debug: stm32: add UART early console support for STM32MP1

Add support of early console for STM32MP1. Default UART instance is UART4,
but other UART instances can be configured by setting physical and virtual
base addresses in menuconfig.

Signed-off-by: Erwan Le Ray <erwan.leray@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>