fs: littlefs: define mount point structures for file systems

Use the devicetree filesystem bindings to populate an fs_mount_t
object that is preconfigured for a particular set of file system
properties on a specified partition.

At this time the mount point data is accessed by reference using the
partition's devicetree node identifier.

Note: While a file system can register itself before its devices
are available, it cannot do the automount.  In this commit the
initialization priority is increased to compensate, but that's not
a long-term solution.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>