docs(fconf): add TB_FW config bindings

Document bindings for TB_FW_CONFIG that are common between platforms.
Since the information this device tree type contains pertains to
firmware specific properties, we do not expect that the document will
cover all uses, nor do we promise backward compatiblity.

Change-Id: I0e850c13b77cc62940ab5020a15bf8e503568ed8
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
diff --git a/docs/components/fconf/index.rst b/docs/components/fconf/index.rst
index 029f324..b8b4519 100644
--- a/docs/components/fconf/index.rst
+++ b/docs/components/fconf/index.rst
@@ -147,3 +147,4 @@
   fconf_properties
   amu-bindings
   mpmm-bindings
+  tb_fw_bindings