How to use fontspec with palatino package?

I use the scrbook class to change the heading font easily:

\documentclass{scrbook}
\usepackage[paperheight=9in,paperwidth=6in]{geometry}
\usepackage{fontspec}
\setmainfont{TeX Gyre Pagella}%% The Palatino from the TeX Gyre Project
\newfontfamily\headingfont{RockwellStd}%% I have only the RockwellStd.otf
\addtokomafont{sectioning}{\headingfont}
\begin{document}
    \chapter{foo}
    \section{bar}
    This text should be in Palatino.
\end{document}