i2c: mux: gpio: remove support for class-based device instantiation

i801 as only user of gpio i2c mux removed support for class-based device
instantiation on muxed busses. Class-based device instantiation is a
legacy mechanism and shouldn't be used in new code, therefore remove
support also here.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>