dt-bindings: iio: adi,ltc2983: Fix 64-bit property sizes

The '/bits/ 64' notation applies the next <> list of values. Another <> list
is encoded as 32-bits by default. IOW, each <> list needs to be preceeded
with '/bits/ 64'.

While the dts format allows this, as a rule we don't mix sizes for DT
properties since all size information is lost in the dtb file.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20220107032026.2408196-1-robh@kernel.org