iio: adc: ad4062: Add GPIO Controller support

iio: adc: ad4062: Add GPIO Controller support

When gp0 or gp1 is not taken as an interrupt, expose them as GPO if
gpio-contoller is set in the devicetree. gpio-regmap is not used
because the GPO static low is 'b101 and static high is 0b110; low state
requires setting bit 0, not fitting the abstraction of low=0 and
high=mask.

Signed-off-by: Jorge Marques <jorge.marques@analog.com>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Acked-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>