dt-bindings:iio:adc:fsl,imx7d-adc yaml conversion

A nice simple binding.  Only real different from txt is that I dropped
some descriptions where the naming of the parameter was self explanatory

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Haibo Chen <haibo.chen@nxp.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Link: https://lore.kernel.org/r/20200909175946.395313-20-jic23@kernel.org