MAINTAINERS: Update mailing list and web page for hwmon subsystem

commit 968ce1b1 upstream.

The old web page for the hwmon subsystem is no longer operational,
and the mailing list has become unreliable. Move both to kernel.org.
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
[ kamal: backport to 3.19-stable: context ]
Signed-off-by: Kamal Mostafa <kamal@canonical.com>