dt-bindings: iio: io-channel-mux: allow duplicate channel, labels

This is needed since an empty channel label is used to indicate an
unused channel, and there can be more that one of those.

Fixes the following problem reported by dtbs_check:
envelope-detector-mux: channels: ['', '', 'sync-1', 'in', 'out', 'sync-2', 'sys-reg', 'ana-reg'] has non-unique elements
	arch/arm/boot/dts/at91-tse850-3.dt.yaml

Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Peter Rosin <peda@axentia.se>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>