dts: Add script to uprev dts/upstream subtree

dts/update-dts-subtree.sh is just a wrapper around git subtree commands.
Usage from the top level U-Boot source tree, run:

$ ./dts/update-dts-subtree.sh pull <release-tag>
$ ./dts/update-dts-subtree.sh pick <commit-id>

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>