cgroupfs: create /sys/fs/cgroup to mount cgroupfs on
We really shouldn't be asking userspace to create new root filesystems.
So follow along with all of the other in-kernel filesystems, and provide
a mount point in sysfs.

For cgroupfs, this should be in /sys/fs/cgroup/  This change provides
that mount point when the cgroup filesystem is registered in the kernel.

Commit: https://github.com/androidarmv6/android_kernel_samsung_bcm21553-common/commit/3877ae5bcea2bcb7f11a1555714bb15f6a109819#diff-c04090c51d3c6700c7128e84c58b1291R3886