dt-bindings: gpu: arm,mali-midgard: add exynos7870-mali compatible

dt-bindings: gpu: arm,mali-midgard: add exynos7870-mali compatible

Exynos7870 SoC uses the ARM Mali T830 GPU, document its compatible
string with the appropriate fallback. The T830 compatible is already
defined in the panfrost driver, but was commented out as it was unused.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Kaustabh Chakraborty <kauschluss@disroot.org>
Link: https://lore.kernel.org/r/20250318-exynos7870-gpu-v2-1-58dc2094dc7f@disroot.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>