Bug 1546595: Two followup fixes for windows10-aarch64 in CI: 1) drop to tier 3; 2) explicitly further limit tests' runs to try + m-c. r=gbrown,jmaher

Differential Revision: https://phabricator.services.mozilla.com/D28623

--HG--
extra : moz-landing-system : lando

taskcluster/ci/test/talos.yml

@@ -243,6 +243,7 @@ talos-motionmark:
     tier:
         by-test-platform:
             windows10-64-ccov/.*: 3
+            windows10-aarch64/.*: 2
             linux64-ccov/.*: 3
             default: 2
     mozharness: