dt-bindings: iio: dac: fix ad3552r gain parameter names

dt-bindings: iio: dac: fix ad3552r gain parameter names

The adi,gain-scaling-p/n values are an inverted log2,
so initial naming was set correctly, but the driver uses just
adi,gain-scaling-p/n, and if fdt is created accordingly with
the fdt bindings documentation, driver fails the probe.

Observing that:
- the Linux driver is the only consumer,
- there are no upstreamed dts nodes related to ad3552r,

the fix to the documentation side is preferred and less-risk.

Fixes: b0a96c5f599e ("dt-bindings: iio: dac: Add adi,ad3552r.yaml")
Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240522150141.1776196-2-adureghello@baylibre.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>