DT: Add documentation for gp2ap020a00f sensor

This patch adds device tree binding documentation
for the gp2ap020a00f proximity/als sensor.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>