dt-bindings: net: mtk-star-emac: add description for new properties

Add description for new properties which will be parsed in driver.

Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>