iio: adc: meson-saradc: mark all meson_sar_adc_data static and const

These are only passed as of_device_id.data and never modified. Thus we
can mark them as static const, just like the of_device_id instances
where they are used.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>