cgroup: allow associate to tmpfs

Allows groups to be mounted at /dev/memcg

Addresses:
avc: denied { associate } for comm="init" name="memcg"
scontext=u:object_r:cgroup:s0 tcontext=u:object_r:tmpfs:s0
tclass=filesystem permissive=0

Bug: 64067152
Test: build
Change-Id: Ic8f641e841fe09c8f7fd487ed67cf0ab4860a1cc