dt-bindings: iio: adi,ltc2983: Add missing quotes on dependencies

With the bracketed list form, any strings with commas have to be quoted
or they are separated.

Fixes: 3986a148

 ("dt-bindings: iio: Add ltc2983 documentation")
Cc: "Nuno Sá" <nuno.sa@analog.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Cc: linux-iio@vger.kernel.org
Acked-by: Nuno Sá <nuno.sa@analog.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Rob Herring <robh@kernel.org>