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>