refactor(st): configure UART baudrate

Add the possibility to configure console UART baudrate, it can be passed
as a command line parameter with STM32MP_UART_BAUDRATE. The default value
remains 115200.

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: I000df70c10b2b4dac1449556596f9820c36cf243
4 files changed