iio: adc: axp20x_adc: Add support for AXP192

The AXP192 is identical to the AXP20x, except for the addition of
two more GPIO ADC channels.
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>