MIPS: have "plain" make calls build dtbs for selected platforms

scripts/package/builddeb calls "make dtbs_install" after executing
a plain make (i.e. no build targets specified). It will fail if dtbs
were not built beforehand. Match the arm64 architecture where DTBs get
built by the "all" target.

Signed-off-by: Cedric Hombourger <Cedric_Hombourger@mentor.com>
[paul.burton@mips.com: s/builddep/builddeb]
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Cc: stable@vger.kernel.org # v4.1+