Docs/kernel-parameters: Add earlycon devicetree usage

This form of the earlycon parameter was added by commit fb11ffe74c794a5
("of/fdt: add FDT serial scanning for earlycon") without documentation.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 945bc0d..7a01cc0 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -973,6 +973,10 @@
 
 	earlycon=	[KNL] Output early console device and options.
 
+			When used with no options, the early console is
+			determined by the stdout-path property in device
+			tree's chosen node.
+
 		cdns,<addr>
 			Start an early, polled-mode console on a cadence serial
 			port at the specified address. The cadence serial port