iio: adc: ab8500-gpadc: Support non-hw-conversion

iio: adc: ab8500-gpadc: Support non-hw-conversion

The hardware conversion mode only exists in the AB8500
version of the chip, as it is lacking in the AB8505 it
will not be in the device tree and we should just not
even try to obtain it.

The driver already contains code to avoid using a
non-existing hardware conversion IRQ at conversion time.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20201218222013.383704-1-linus.walleij@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>