iio: adc: meson-saradc: Meson8 and Meson8b do not have REG11 and REG13

The Meson GXBB and newer SoCs have a few more registers than the older
Meson8 and Meson8b SoCs.
Use a separate regmap config to limit the older SoCs to the DELTA_10
register.

Fixes: 6c76ed31 ("iio: adc: meson-saradc: add Meson8b SoC compatibility")
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>