doc: dtbinding: Add file system firmware loader binding document

Add a document to describe file system firmware loader binding
information.

Signed-off-by: Tien Fong Chee <tien.fong.chee@intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/device-tree-bindings/chosen.txt b/doc/device-tree-bindings/chosen.txt
index da7b4e6..86c533a 100644
--- a/doc/device-tree-bindings/chosen.txt
+++ b/doc/device-tree-bindings/chosen.txt
@@ -83,3 +83,24 @@
 
 You should not define this property yourself in the device-tree, as it
 may be overwritten without warning.
+
+firmware-loader property
+------------------------
+Multiple file system firmware loader nodes could be defined in device trees for
+multiple storage type and their default partition, then a property
+"firmware-loader" can be used to pass default firmware loader
+node(default storage type) to the firmware loader driver.
+
+Example
+-------
+/ {
+	chosen {
+		firmware-loader = &fs_loader0;
+	};
+
+	fs_loader0: fs-loader@0 {
+		u-boot,dm-pre-reloc;
+		compatible = "u-boot,fs-loader";
+		phandlepart = <&mmc 1>;
+	};
+};