A Riverside man who fatally stabbed his 62-year-old mother during an argument at her Riverside apartment pleaded guilty Thursday to second-degree murder. Otis James Porter, 41, admitted the charge, along with a sentence-enhancing allegation of using a deadly weapon — a knife — in the commission of a felony, under a plea agreement with the Riverside County District Attorney’s Office. No charges were dismissed under the deal, which was announced j…