Add github workflow definition

Add a file to run `make check' on github.
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>

.github/workflows/ci.yaml

+---
+name: CI
+# yamllint disable-line rule:truthy
+on: [push]
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Install dependencies and check
+        run: |
+          sudo apt install flake8
+          make check