iio: light: Add DT bindings for GP2AP002

This adds device tree bindings for the GP2AP002 light
and proximity sensor.

As with other early proximity sensors (~2010) the light
sensor and proximity sensors were combined into a single
component.

Cc: Stephan Gerhold <stephan@gerhold.net>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Paweł Chmiel <pawel.mikolaj.chmiel@gmail.com>
Cc: Jonathan Bakker <xc-racer2@live.ca>
Cc: Oskar Andero <oskar.andero@gmail.com>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>