misc: fs_loader: Add function to get the chosen loader

The fs_loader device is used to pull in settings via the chosen node.
However, there was no library function for this, so arria10 was doing it
explicitly. This function subsumes that, and uses ofnode_get_chosen_node
instead of navigating the device tree directly. Because fs_loader pulls
its config from the environment by default, it's fine to create a device
with nothing backing it at all. Doing this allows enabling
CONFIG_FS_LOADER without needing to modify the device tree.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>