dt-bindings:iio:adc:fsl,imx25-gcq yaml conversion

This binding has a few corners that would have been done different today
but hopefully the yaml schema captures the constraints correctly.

The child node names are not constrained hence the fairly open regexp.
I've also documented the defaults for the two references that the
driver seems to use and copied the value descriptions from the header
because I think they should be in the dt-binding itself.

This is part of a general effort to convert all the IIO bindings
over to yaml

Unfortunately I don't have a current address for Markus, so
have put myself as the maintainer for this binding until someone else
steps up!

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Link: https://lore.kernel.org/r/20200909175946.395313-21-jic23@kernel.org