John Legend has some serious star power on his side.The performer revealed on Monday that his celeb mentor for the upcoming season will be none other than… Brandy!«Her track record as a successful artist speaks for itself and we've been loving her and cherishing her as an artist of our culture for such a long time,» Legend raved when the two sat down with ET. «What I tell her and what is so resonant to me about her is not only has she had a great career and had lots of hits, but singers love