iio: light: gp2ap020a00f: Select REGMAP_I2C

This patch adds missing 'select' statement for gp2ap020a00f driver.
Without regmap_i2c, we get the following error when loading the module:
Unknown symbol devm_regmap_init_i2c.

Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
Reviewed-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>