cmd: consider multiplexing in coninfo

If console multiplexing in enabled (CONFIG_CONSOLE_MUX=y), the output of
the coninfo command should show the file association (stdin, stderr,
stdout) for all devices not only the default ones.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>