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>
diff --git a/configs/turris_omnia_defconfig b/configs/turris_omnia_defconfig
index bce14cc..ba635fe 100644
--- a/configs/turris_omnia_defconfig
+++ b/configs/turris_omnia_defconfig
@@ -33,6 +33,7 @@
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_BOOTDELAY=3
 CONFIG_USE_PREBOOT=y
+CONFIG_CONSOLE_MUX=y
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_DISPLAY_BOARDINFO_LATE=y
@@ -103,6 +104,7 @@
 CONFIG_DM_RTC=y
 CONFIG_RTC_ARMADA38X=y
 CONFIG_SCSI=y
+CONFIG_SERIAL_PROBE_ALL=y
 CONFIG_SPL_DEBUG_UART_BASE=0xd0012000
 CONFIG_DEBUG_UART_SHIFT=2
 CONFIG_SYS_NS16550=y