configs: stm32mp13: Add support for baudrates higher than 115200

On STM32MP13x STMicroelectronics boards, the UART can reliably go up to
4000000 bauds when connected to the external ST-LINKV3.

This patch adds the support of higher baudrates on STMicroelectronics
STM32MP13x boards with ST-LINKV3.

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