uboot-imx

doc: Add new doc for file system firmware loader driver model

Provide information about

- overview of file system firmware loader driver model
- describe storage device and partition in device tree source
- describe fie system firmware loader API

Signed-off-by: Tien Fong Chee <tien.fong.chee@intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>