iio: light: gp2ap020a00f: Swap of.h for mod_devicetable.h + drop of_match_ptr

Also drops ifdef protections for CONFIG_OF.

Enables probing via ACPI PRP0001 and removes an example that might be
cut and paste into new drivers.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>