hwmon: (vexpress) Avoid creating non-existing attributes

The 'label' attribute was always created but returned -ENOENT
if there is no label and such behaviour is undefined from
libsensors' point of view.

Fixed by providing is_visible method in the attributes group,
so the attribute is not created at all when unnecessary.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>