cgroup: Fix automatic disabling of cgroup "memory"

Some Raspberry Pis have limited RAM and most users won't use the
cgroup memory support so it is disabled by default. The previous
implementation relied on the memory cgroup being encountered first,
which is not guaranteed to be the case. Change the implementation
to be registration-order-agnostic.

See: https://github.com/raspberrypi/issues/1950



Signed-off-by: Phil Elwell <phil@raspberrypi.org>