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>
diff --git a/arch/arm/mach-stm32mp/cmd_stm32prog/Makefile b/arch/arm/mach-stm32mp/cmd_stm32prog/Makefile
index 14f7227..548a378 100644
--- a/arch/arm/mach-stm32mp/cmd_stm32prog/Makefile
+++ b/arch/arm/mach-stm32mp/cmd_stm32prog/Makefile
@@ -5,4 +5,5 @@
 
 obj-y += cmd_stm32prog.o
 obj-y += stm32prog.o
+obj-y += stm32prog_serial.o
 obj-y += stm32prog_usb.o