dt-bindings:iio:dac:microchip,mcp4725: fix properties for mcp4726

dt-bindings:iio:dac:microchip,mcp4725: fix properties for mcp4726

The vdd-supply property is optional if vref-supply is provided for
mcp4726.

Also the microchip,vref-buffered makes sense only if vref-supply is
specified.

Spotted by Jonathan during conversion to yaml.

Reported-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Tomas Novotny <tomas@novotny.cz>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201216101316.1403-1-tomas@novotny.cz
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>