i2c: mux: pinctrl: remove platform_data

No platform (at least no upstreamed platform) has ever used this
platform_data. Just drop it and simplify the code.

Reviewed-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Peter Rosin <peda@axentia.se>