workflows: We all love checkpatch, so add it to the CI workflows

This is currently running on defaults, so the --strict desired
for media drivers and similar won't be observed. That may be
possible to add later.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>

.github/workflows/checkpatch.yml

@@ -0,0 +1,18 @@
+name: Advisory checkpatch review
+on: [pull_request]
+
+jobs:
+  review:
+    name: checkpatch review
+    runs-on: ubuntu-latest
+    steps:
+    - name: 'Calculate PR commits + 1'
+      run: echo "PR_FETCH_DEPTH=$(( ${{ github.event.pull_request.commits }} + 1 ))" >> $GITHUB_ENV
+    - uses: actions/checkout@v3
+      with:
+        ref: ${{ github.event.pull_request.head.sha }}
+        fetch-depth: ${{ env.PR_FETCH_DEPTH }}
+    - name: Copy checkpatch.conf
+      run: cp ${{github.workspace}}/.github/workflows/ci_checkpatch.conf ${{github.workspace}}/.checkpatch.conf
+    - name: Run checkpatch review
+      uses: webispy/checkpatch-action@v9

.github/workflows/ci_checkpatch.conf

@@ -0,0 +1,4 @@
+--no-tree
+--ignore FILE_PATH_CHANGES
+--ignore GIT_COMMIT_ID
+--ignore SPDX_LICENSE_TAG