dt-bindings: gpu: mali-bifrost: Fix power-domain-names validation

Commit ("dt-bindings: gpu: mali-bifrost: Add Mediatek MT8183")
incorrectly introduced power domain names for MT8183, causing
validation issues.

Add power-domain-names to the base schema, allowing a maximum of
five elements; since platforms having a single power domain don't
need any actual domain name, disallow that for each sub-schema.

Fixes: a7a596cd ("dt-bindings: gpu: mali-bifrost: Add Mediatek MT8183")
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: Chen-Yu Tsai <wenst@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230316102041.210269-4-angelogioacchino.delregno@collabora.com