hwmon: (adt7475) Add an entry in MAINTAINERS

As I've just done a lot of changes to the adt7475 driver, I volunteer
to maintain it for the year to come.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Jordan Crouse <jordan@cosmicpenguin.net>
Cc: "Darrick J. Wong" <djwong@us.ibm.com>