Add fs.c under SPL as well as it is needed for fs_loader

Signed-off-by: Keerthy <j-keerthy@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
[trini: Add as obj-$(CONFIG_FS_LOADER) for non-SPL_FRAMEWORK builds]
Signed-off-by: Tom Rini <trini@konsulko.com>