arm: mvebu: turris_omnia: Allow to use second serial port

Turris Omnia has two serial ports. Both are already specified in device
tree file. But U-Boot by default does not allow to use more than one serial
port unless CONFIG_SERIAL_PROBE_ALL is not enabled.

After enabling CONFIG_SERIAL_PROBE_ALL, U-Boot see also second serial port
(but is inactive by default):

    => coninfo
    List of available devices:
    serial@12000 00000007 IO stdin stdout stderr
    serial@12100 00000007 IO

To allow simultaneously to use more input / output devices it is needed to
enable CONFIG_CONSOLE_MUX option.

With CONFIG_CONSOLE_MUX it is possible to call:

    => setenv stdout 'serial@12000,serial@12100'

And U-Boot output is then visible on both serial ports.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>