There is a place called Martorano, near Cesena in mainland Italy. Though this is unlikely to be the origin of the Sicilian surname it does raise the possiblity of a place-name origin.
Alternatively,
www.melegnano.net/cognomi lists the names Martori, Martorini, Martorelli and Martorello. The suggestion is that the basis of these names is a dialect word MARTOR, which is explained as "sempliciotto" (simpleton?).