dt-bindings: gpu: mali-bifrost: Add RK3568 compatible

The Rockchip RK3568 SoC has a Bifrost Mali-G52 GPU,
add a compatible string for it.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Link: https://lore.kernel.org/r/20210805025948.10900-3-ezequiel@collabora.com


Signed-off-by: Rob Herring <robh@kernel.org>