dt-bindings:iio:dac:microchip,mcp4725 yaml conversion

I'm not sure vdd-supply absolutely has to be provided if vref-supply
is, but as the previous binding docs stated it was required it seems
reasonable to leave it as such.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Tomas Novotny <tomas@novotny.cz>
Link: https://lore.kernel.org/r/20201031134110.724233-21-jic23@kernel.org