i8k: Add support for Dell Precision 490 and Latitude D520

Both systems need non-standard parameters for fan multiplier
and maximum fan speed.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: Andreas Mohr <andi@lisas.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>