dt-bindings: display: sunxi: Fix compatible

Commit f5a98bfe ("dt-bindings: display: Convert Allwinner display
pipeline to schemas") introduced a YAML schema for the Allwinner TCON DT
binding, but the H6 TCON-TV compatible was mistakenly set to fallback on
the A83t's, while the initial documentation and the DT are using R40's.

Fix that.

Fixes: f5a98bfe ("dt-bindings: display: Convert Allwinner display pipeline to schemas")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200210100455.78695-1-maxime@cerno.tech