ARM: dts: kirkwood: provide template for RS-232/485 configuration for OpenRD

Some OpenRD boards have RS-232 and RS-486 connectors wired, but using them
needs a custom DTB as the current DTB configures SD card slot instead.

This patch adds documentation into the DTS on how to change
the configuration.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>