build: Don't remove dts/ directory on clean

This patch fixes an issue where any make target executing the the clean
from the root source directory would remove the dts directory.  This is
not necessary and can cause build issues on following make commands.

ZEP-1968

Change-Id: I2cc751d8fd24bd77e425860686b66644ade44eeb
Signed-off-by: Andy Gross <andy.gross@linaro.org>