backlight: adp8860: Move header file out of I2C realm

include/linux/i2c is not for client devices.

Move the header file to a more appropriate location.

Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>