dt-bindings: iio: adc: maxim,max11100 yaml conversion

Straight forward conversion for this SPI ADC.
Added limits on spi-max-frequency from datasheet (0.1 to 4.8MHz)

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Rob Herring <robh@kernel.org>