ARM: debug: stm32: add UART early console support for STM32H7

Add support of early console for STM32H7. 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>