dt-bindings:iio:adc:ad7280a: Add binding

Add a binding for this Lithium Ion Battery monitoring chip/chain of chips
as it is now clean and ready to move out of staging.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://lore.kernel.org/r/20220206190328.333093-17-jic23@kernel.org