Switch devicetree-rebasing to kernel.googlesource.com mirror

CI builds have been failing lately with:
Error: fatal: unable to access
'https://git.kernel.org/pub/scm/linux/kernel/git/devicetree/devicetree-rebasing.git/':
Failed to connect to git.kernel.org port 443 after
134198 ms: Couldn't connect to server

Signed-off-by: Mario Bălănică <[email protected]>

Author: mariobalanica

.gitmodules

@@ -13,7 +13,7 @@
 	branch = rk3588
 [submodule "devicetree/mainline/upstream"]
 	path = devicetree/mainline/upstream
-	url = https://git.kernel.org/pub/scm/linux/kernel/git/devicetree/devicetree-rebasing.git
+	url = https://kernel.googlesource.com/pub/scm/linux/kernel/git/devicetree/devicetree-rebasing.git
 [submodule "edk2-platforms"]
 	path = edk2-platforms
 	url = https://github.com/tianocore/edk2-platforms.git