i2c: i2c-mux-gpio: rename i2c-gpio-mux to i2c-mux-gpio

The rename did the wrong thing for this documentation file all those
years ago. Fix that as well as the neglected rename of the platform
data structure.

Fixes: e7065e20 ("i2c: Rename last mux driver to standard pattern")
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>