net: hisilicon: fix binding document of mdio

This patch explains the occasion of "hisilcon,mdio" and
"hisilicon,hns-mdio" according to Arnd's comments.
and reformat it according to comments from Rob<robh@kernel.org>.

Signed-off-by: huangdaode <huangdaode@hisilicon.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>