dt-bindings: Add compatible for Mali Valhall (JM)

From the kernel's perspective, (pre-CSF, "Job Manager") Valhall is more
or less compatible with Bifrost, although they differ to userspace. Add
a compatible for Valhall to the existing Bifrost bindings documentation.

As the first SoC with a Valhall GPU receiving mainline support, add a
specific compatible for the MediaTek MT8192, which instantiates a
Mali-G57.

v2: Change compatible to arm,mali-valhall-jm (Daniel Stone).

CC: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220525145754.25866-2-alyssa.rosenzweig@collabora.com