i8k: Add support for Dell XPS 13

XPS 13 does not support turbo speed, so its initialization data
matches that of XPS M140. Make XPS initialization data generic,
and add support for XPS 13.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>