iio: adc: sun4i-gpadc: select REGMAP_IRQ

We can't call regmap_irq_get_virq() unless the regmap-irq support
is enabled:

drivers/iio/adc/sun4i-gpadc-iio.o: In function `sun4i_irq_init':
sun4i-gpadc-iio.c:(.text+0x59c): undefined reference to `regmap_irq_get_virq'

I came across this in a randconfig build now, but I guess this is
a much older problem that I simply never hit before because regmap_irq
is usually enabled by one of the other drivers that need it.

Fixes: d1caa990

 ("iio: adc: add support for Allwinner SoCs ADC")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>