fs: provide access to DT-defined mount structures

Use a devicetree fstab entry node identifier to provide a unique
identifier for a fs_mount_t structure that is defined with everything
necessary to mount the file system associated with a partition.

The fs_mount_t structure will be defined in the implementation for
each file system type.

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