fs: enable fstab devicetree integration for fatfs

In order to use fstab with fatfs in addition to
littlefs a new driver fstab,fatfs was introduced
containing the required logic to detect and if
needed automount fatfs devices specified in fstab.
Additionally as the partition phandle is not needed
by fatfs and currently is specific to littlefs it
was moved from the common dts binding into the
littlefs binding.

Signed-off-by: Carlo Kirchmeier <carlo.kirchmeier@zuehlke.com>
Co-authored-by: Carlo Kirchmeier <carlo.kirchmeier@zuehlke.com>
Co-authored-by: Dominik Ermel <dominik.ermel@nordicsemi.no>