i2c: kempld: deprecate class based instantiation

Warn users that class based instantiation is going away soon in favour
of more robust probing and faster bootup times.

Class based instantiation has already been removed for other controllers
and it makes absolutely sense to do it for this one too.

Signed-off-by: Michael Brunner <michael.brunner@kontron.com>
Acked-by: Ingmar Klein <ingmar.klein@kontron.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>