cgroup: Retain support for cgroup_memory=1

As part of a misguided fix for the automatic disabling of cgroup
memory, a new kernel parameter "cgroup_memory" was added. This
extra parameter is unnecessary now that implicit memory cgroup
disabling works correctly, but some unfortunate users may have
come to rely on it.

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



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