i2c: mux: pinctrl: mention correct module name in Kconfig help text

Kconfig says the resulting module is pinctrl-i2cmux, but the module when
built is i2c-mux-pinctrl.

Fixes: ae58d1e4 ("i2c: Add generic I2C multiplexer using pinctrl API")
Signed-off-by: Chris Gorman <chrisjohgorman@gmail.com>
Signed-off-by: Peter Rosin <peda@axentia.se>