dt-bindings: gpu: mali-bifrost: Document RZ/V2L SoC

The Renesas RZ/V2L SoC (a.k.a R9A07G054) has a Bifrost Mali-G31 GPU,
add a compatible string for it.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220308211543.3081-1-prabhakar.mahadev-lad.rj@bp.renesas.com