dt-bindings: net: mediatek: add condition to property mediatek, pctl

The property "mediatek,pctl" is only required for SoCs such as MT2701 and
MT7623, so adding a few words for stating the condition.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>