w1: add support for DS2438 Smart Battery Monitor

Detailed information about support and provided sysfs files
in my next commit which creates a documentation file:
Documentation/w1/slaves/w1_ds2438

Signed-off-by: Mariusz Bialonczyk <manio@skyboo.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>