misc: fs_loader: Switching private data allocation to DM auto allocation

Switching private data manual allocation to driver model auto allocation
so users no longer need to deallocate themself because this would be
deallocated by driver model when the device is no longer required.

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