dt-bindings: gpu: mali-bifrost: Document RZ/G2L support

The Renesas RZ/G2{L, LC} SoC (a.k.a R9A07G044) has a Bifrost Mali-G31 GPU,
add a compatible string for it.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://lore.kernel.org/r/20211208104026.421-2-biju.das.jz@bp.renesas.com


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