ARM: debug: stm32: add UART early console configuration for STM32F7

Early console is hardcoded on USART1 in current implementation.
With this patch, default UART instance is USART1, 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>