Given this passage on page 9 under Duration: Concentration first sentence of the second paragraph.
Concentrating on a discipline follows the same rules as concentrating on a spell.
I would rule, that you cannot concentrate on a discipline while raging.
But aside from that, disciplines are strictly no spells, so RAW you are able to use them while in rage. (Unless the Discipline says 'you cast some spell'.)
Rules as Intended:
Mike Mearls mentioned in a tweet, that it shouldn't be possible:
@Spiritmaster111: @mikemearls Also, can a Mystic/Barbarian multiclass use disciplines while raging? Rage only mentions casting and concentrating on spells.
@mikemearls: @Spiritmaster111 should also apply to psionics - thanks for pointing that out