dt-bindings: iio: admv1014: make all regs required

Make the regulators required in the dt bindings.

Despite the fact that the datasheet is not explicit enough, all the
specifications of the part are built around these pins being supplied.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230731144404.389255-1-antoniu.miclaus@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>