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: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/device-tree-bindings/config.txt b/doc/device-tree-bindings/config.txt
new file mode 100644
index 0000000..ebb9bd4
--- /dev/null
+++ b/doc/device-tree-bindings/config.txt
@@ -0,0 +1,14 @@
+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