Makefile: remove hardcoded device tree source directory

Some boards that choose to utilize the OF_UPSTREAM directory for their
device tree files will need to specify that directory instead of the
traditional arch/$(ARCH)/dts/* path.

Include the correct path to the board's dtbs depending on if OF_UPSTREAM
is selected or not.

Signed-off-by: Bryan Brattlof <bb@ti.com>
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Tested-by: Fabio Estevam <festevam@gmail.com>