dt-bindings:iio:adc:adi,ad7768-1 yaml conversion

Fairly straight conversion.  The one oddity in the original binding
is that spi-cpha and spi-cpol were not marked as required, but were
in the example.  Looking at the datasheet, there isn't any documented
flexibility in the possible SPI modes, so I have moved these to requires.
For spi-max-frequency I have gone the other way.  I absolutely agree
that it is good to specify this in the dt-binding, but it's not
strictly required.

As Stefan's email is bouncing I have gone with Michael as maintainer
of this one as it falls under the ADI catch all entry in MAINTAINERS.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Link: https://lore.kernel.org/r/20200909175946.395313-16-jic23@kernel.org