iio: light: add DT bindings for Capella CM3605

This adds device tree bindings for the Capella Microsystems CM3605
ambient light sensor and short range proximity sensor.

Cc: devicetree@vger.kernel.org
Cc: Capella Microsystems <capellamicro@gmail.com>
Cc: Kevin Tsai <ktsai@capellamicro.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>