ARC: [arcfpga] stdout-path now suffices for earlycon/console

With recent improvements to serial/of core from Grant and Rob,
stdout-path alone suffices for setting up earlycon/console.

arc_uart driver is already equipped to handle that, switch the DT now.

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