iio: adc: mcp320x: Deprecated compatible strings with no vendor prefix

The Microchip Analog to Digital Converter (ADC) Device Tree binding
documents compatible strings with no vendor prefix. Since it should
compatible strings with also a vendor, add these to the binding doc
and mark the old ones as deprecated.

The driver says that the device is from Microchip Technology which
is listed in Documentation/devicetree/bindings/vendor-prefixes.txt
so use the documented prefix.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Michael Welling <mwelling@ieee.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>