ARM: 7774/1: Fix dtb dependency to use order-only prerequisites

The %.dtb dependency is specified to depend on the PHONY "scripts".
That means that it'll build every time even if the underlying dtb file
hasn't been touched.  Use an order-only prerequisites to fix this.
Also mark "dtbs" as PHONY for correctness.

This was broken in (70b0476a ARM: 7513/1: Make sure dtc is built before
running it).

Reported-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Doug Anderson <dianders@chromium.org>
Acked-by: Olof Johansson <olof@lixom.net>
Reviewed-by: David Brown <davidb@codeaurora.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>