dt-bindings:iio:adc:fsl,vf610-adc conversion to yaml.

A simple conversion of this freescale ADC binding from txt to yaml.
For maintainer I went with Fugang Duan as the original author of the
binding. Would be great to have confirmation of this.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Fugang Duan <fugang.duan@nxp.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/20200909175946.395313-2-jic23@kernel.org