i2c: i2c-cbus-gpio: Add vendor prefix to retu node in example

The example contains a device node for a retu device, but
its compatible string doesn't have a vendor prefix.

While being there, drop the -mfd suffix since isn't correct.

Signed-off-by: Javier Martinez Canillas <javier@dowhile0.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Acked-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>