cgroup: Add kernel param to enable memory group

cgroup support is useful, but enabling the "memory" cgroup has a 32-byte
overhead per page. Arrange that "memory" cgroup is disabled by default
unless the "cgroup_memory" parameter is set to a true value.

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



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