dt-bindings: iio: dac: AD5570R fix bindings errors

Replaced num property with reg property, fixed errors
reported by dt-binding-check.

Fixes: ea52c212 ("dt-bindings: iio: dac: Add docs for AD5770R DAC")
Signed-off-by: Alexandru Tachici <alexandru.tachici@analog.com>
[robh: Fix required property list, fix Fixes tag]
Signed-off-by: Rob Herring <robh@kernel.org>