If you actually love Psychology, I say go for it, but only do it if that's what your passionate about. The new MCAT will have psychology in it, but to focus your major solely for the purpose of that additional MCAT knowledge would be overkill. Keep in mind if you major in something you'll be required to take a ton of upper-division coursework in that major, much of which is going to be out of scope for the MCAT exam. It would be much wiser to pick something you love, and add an into to psychology course along with maybe one or two additional classes if any.