dts, arm/arm64: Remove dtbs build rules in sub-makes

Add dtb files to build targets and let kbuild handle them. Thus,
special dtbs rules can be removed. This eases Makefiles and the
implementation of the support of vendor dtb subdirectories.

Signed-off-by: Robert Richter <rrichter@cavium.com>
(cherry picked from commit d38726c485d3fd6e9f2ebd1adaef0c3ead48a5ac)
Signed-off-by: Kevin Hilman <khilman@linaro.org>