serial: timberdale: Depend on X86_32

As far as I know the Timberdale chip was only used as a companion for
Intel Atom E600 series processors. As such, its drivers are only
useful on X86_32.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>