doc: move documentation for /config node into a separate file

This moves the description of the /config node from README.fdt-control
into a separate file doc/device-tree-bindings/config.txt.
Signed-off-by: NPhilipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: NSimon Glass <sjg@chromium.org>

doc/README.fdt-control

@@ -168,22 +168,6 @@ After board configuration is done, fdt supported u-boot can be build in two ways
     $ make DEVICE_TREE=<dts-file-name>
 
 
-Configuration Options
----------------------
-
-A number of run-time configuration options are provided in the /config node
-of the control device tree. You can access these using fdtdec_get_config_int(),
-fdtdec_get_config_bool() and fdtdec_get_config_string().
-
-Available options are:
-
-silent-console
-	If present and non-zero, the console is silenced by default on boot.
-
-no-keyboard
-	Tells U-Boot not to expect an attached keyboard with a VGA console
-
-
 Limitations
 -----------
 

doc/device-tree-bindings/config.txt

+The /config node (Configuration Options)
+----------------------------------------
+
+A number of run-time configuration options are provided in the /config node
+of the control device tree. You can access these using fdtdec_get_config_int(),
+fdtdec_get_config_bool() and fdtdec_get_config_string().
+
+Available options are:
+
+silent-console
+	If present and non-zero, the console is silenced by default on boot.
+
+no-keyboard
+	Tells U-Boot not to expect an attached keyboard with a VGA console