of: Enable console on serial ports specified by /chosen/stdout-path

If the devicetree specifies a serial port as a stdout device, then the
kernel can use it as the default console if nothing else was selected on
the command line. For any serial port that uses the uart_add_one_port()
feature, the uart_add_one_port() has all the information needed to
automatically enable the console device, which is what this patch does.

With this change applied, a device tree platform can be booted without
any console= parameters on the command line and the kernel will still be
able to determine its console.

Tested on QEMU Versatile model and i.MX

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Tested-by: Sascha Hauer <s.hauer@pengutronix.de>