dt-bindings:iio:adc:maxim,max1027: Pull out to separate binding doc.

The afe/voltage-divider.yaml example uses this device with 2 properties
not provided by trivial-devices.yaml (spi-max-frequency and #io-channel-cells)

Solve that by creating a more specific binding doc.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Philippe Reynes <tremyfr@yahoo.fr>
Link: https://lore.kernel.org/r/20201031181242.742301-8-jic23@kernel.org