arm64: dts: mt8183-pumpkin: Couple VGPU and VSRAM_GPU regulators

Add coupling for these regulators, as they have a strict voltage output
relation to satisfy in order to ensure GPU stable operation.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Tested-by: Chen-Yu Tsai <wenst@chromium.org>
Link: https://lore.kernel.org/r/20230301095523.428461-5-angelogioacchino.delregno@collabora.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>