In the current builds of ML for 6d there's no bitrate control options like in the eos-m builds.
I don't now the exact reasons, but my guess is that there's no need for it because the 6d can do ALL-i recording in Canon menu.
But if you really want to try, I remember some builds in 2013 and maybe begin 2014 that had those bitrate setting menu in ML on the 6d.
For all 6d builds: (click in the left column build history on
'more' for really old builds.
http://builds.magiclantern.fm/jenkins/job/6D.116/Try some late 2013 builds, I believe they had the bitrate options in it.
But beware that these are for firmware 1.1.3 on the 6d and not as the latest builds for 1.1.6 firmware.
So maybe you have to downgrade to firmware 1.1.3