An img directory exists for the Pistchio SoC device tree but the
directory itself isn't in the dts Makefile meaning the dtbs never get
built.

Fixes: daa10170 ("MIPS: DTS: img: add device tree for Marduk board")
Signed-off-by: NIan Pozella <Ian.Pozella@imgtec.com>
Reviewed-by: NRahul Bedarkar <Rahul.Bedarkar@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/15309/Signed-off-by: NJames Hogan <james.hogan@imgtec.com>