dt-bindings: gpu: add bindings for the ARM Mali Bifrost GPU

Add the bindings for the Bifrost family of ARM Mali GPUs.

The Bifrost GPU architecture is similar to the Midgard family,
but with a different Shader Core & Execution Engine structures.

Bindings are based on the Midgard family bindings, but the inner
architectural changes makes it a separate family needing separate
bindings.

The Bifrost GPUs are present in a number of recent SoCs, like the
Amlogic G12A Family, and many other vendors.
The Amlogic vendor specific compatible is added to handle the
specific IP integration differences and dependencies.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
[narmstrong: fixed small typo in compatible description]
Link: https://patchwork.freedesktop.org/patch/msgid/20190401080949.14550-1-narmstrong@baylibre.com