ARM: stm32: Set stdio to serial on DH STM32MP15xx DHSOM

In case CONSOLE_MUX and SYS_CONSOLE_IS_IN_ENV are enabled, the console
stdin, stdout, stderr must be defined in environment. Define the default
settings to fix the following warning on boot:

"
In:    No input devices available!
Out:   No output devices available!
Err:   No error devices available!
"

Sort the default environment as well.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>