This should do the trick. Just add this to your `.emacs`

    (defun save-all-and-quit ()
      (interactive)
      (save-some-buffers t)
      (kill-emacs))
    (global-set-key (kbd "S-; x") 'save-all-and-quit)

This is considering you want to save all buffers without prompting interactively for `yes` or `no` for all the unsaved buffers.