lm832x_key_event() is specific go LM832x devices, not to the
I2C bus API. Move it out of "i2c.h" to a new header.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>