metadata: Create DIR_METADATA for the zephyr/hal_stm32 repo

Add DIR_METADATA for the third party Zephyr hal_stm32 repository.

BUG=none
TEST=dirmd validate ./DIR_METADATA

Change-Id: Ie67c328f20409dfccfb023233a14dd8b444d74ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/hal_stm32/+/4943992
Reviewed-by: Fabio Baltieri <fabiobaltieri@google.com>
Tested-by: Keith Short <keithshort@chromium.org>
Auto-Submit: Keith Short <keithshort@chromium.org>
Commit-Queue: Fabio Baltieri <fabiobaltieri@google.com>