toolchain.gccarmemb: set DTC for building targets that use devicetrees

We default DTC to /usr/bin/dtc so we can build targets that are using
devicetree and not using the zephyr SDK (which provides a version of
dtc).

Change-Id: I84c3e3181eda75b4da715e16bbde2e67f28785e7
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>