iio: accel: kxsd9: Add I2C transport

This adds I2C regmap transport for the KXSD9 driver.
Tested on the KXSD9 sensor on the APQ8060 Dragonboard.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>