drm/bridge: Add sii902x DT bindings doc

Add Sii9022 DT bindings description.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
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Changes since v6:
- make 'reset-gpios' optional

Changes since v1:
- rename doc file
- s/sil902/sii902/