stm32mp: stm32prog: add serial link support

Add a support of UART, using the same protocol than MCU STM32.

See "AN5275: USB DFU/USART protocols used in STM32MP1 Series
bootloaders" for details.

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