MAINTAINERS: Add myself as i8k maintainer

I have a couple of Dell laptops running Linux, so I have a vested
interest to keep this driver supported.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>