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>

dts/update-dts-subtree.sh

@@ -0,0 +1,48 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0+
+#
+# Copyright 2024 Linaro Ltd.
+#
+# 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>
+#
+# The script will pull changes from devicetree-rebasing repo into U-Boot
+# as a subtree located as <U-Boot>/dts/upstream sub-directory. It will
+# automatically create a squash/merge commit listing the commits imported.
+
+set -e
+
+merge_commit_msg=$(cat << EOF
+Subtree merge tag '$2' of devicetree-rebasing repo [1] into dts/upstream
+
+[1] https://git.kernel.org/pub/scm/linux/kernel/git/devicetree/devicetree-rebasing.git/
+EOF
+)
+
+remote_add_and_fetch() {
+    if ! git remote get-url devicetree-rebasing 2>/dev/null
+    then
+        echo "Warning: Script automatically adds new git remote via:"
+        echo "    git remote add devicetree-rebasing \\"
+        echo "        https://git.kernel.org/pub/scm/linux/kernel/git/devicetree/devicetree-rebasing.git"
+        git remote add devicetree-rebasing \
+            https://git.kernel.org/pub/scm/linux/kernel/git/devicetree/devicetree-rebasing.git
+    fi
+    git fetch devicetree-rebasing master
+}
+
+if [ "$1" = "pull" ]
+then
+    remote_add_and_fetch
+    git subtree pull --prefix dts/upstream devicetree-rebasing \
+        "$2" --squash -m "${merge_commit_msg}"
+elif [ "$1" = "pick" ]
+then
+    remote_add_and_fetch
+    git cherry-pick -x --strategy=subtree -Xsubtree=dts/upstream/ "$2"
+else
+    echo "usage: $0 <op> <ref>"
+    echo "  <op>     pull or pick"
+    echo "  <ref>    release tag [pull] or commit id [pick]"
+fi