doc: driver-model: Convert fs_firmware_loader.txt to reST

Convert plain text documentation to reStructuredText format and add
it to Sphinx TOC tree. No essential content change.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/driver-model/index.rst b/doc/driver-model/index.rst
index 71ad062..6b8f181 100644
--- a/doc/driver-model/index.rst
+++ b/doc/driver-model/index.rst
@@ -8,4 +8,5 @@
 
    design
    fdt-fixup
+   fs_firmware_loader
    migration