iio: adc: meson-saradc: fix the clock frequency on Meson8 and Meson8b

GX SoCs use a 1.2 MHz ADC clock, while the older SoCs use a 1.14 MHz
clock.

A comment in the driver from Amlogic's GPL kernel says that it's
running at 1.28 MHz. However, it's actually programming a divider of
20 + 1. With a XTAL clock of 24 MHz this results in a frequency of
1.14 MHz. (their calculation might be based on a 27 MHz XTAL clock,
but this is not what we have on the Meson8 and Meson8b SoCs).

The ADC was still working with the 1.2MHz clock. In my own tests I did
not see a difference between 1.2 and 1.14 MHz (regardless of the clock
frequency used, the ADC results were identical).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>