serial/arc: Enable DT based earlycon

This allows a param less earlycon to pick up the earlyconsole from
chosen/stdout-path

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>