MIPS: DTS: Add img directory to Makefile

MIPS: DTS: Add img directory to Makefile

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: daa10170da27 ("MIPS: DTS: img: add device tree for Marduk board")
Signed-off-by: Ian Pozella <Ian.Pozella@imgtec.com>
Reviewed-by: Rahul 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: James Hogan <james.hogan@imgtec.com>