dt-bindings: sunxi: Fix the pinecube compatible

Commit 6ab48105 ("ARM: dts: s3: pinecube: align compatible property
to other S3 boards") changed the pinecube compatible to make it similar
to the other S3 boards we have, but failed to update the bindings
documentation.

Fixes: 6ab48105 ("ARM: dts: s3: pinecube: align compatible property to other S3 boards")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Jernej Skrabec <jernej.skrabec@siol.net>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Link: https://lore.kernel.org/r/20210114113538.1233933-1-maxime@cerno.tech