mfd: retu: Drop -mfd suffix from I2C device ID name

It's not correct to encode the subsystem in the I2C device name, so
drop the -mfd suffix. To maintain bisect-ability, change driver and
platform code / DTS users in the same patch.

Suggested-by: Lee Jones <lee.jones@linaro.org>
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>