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

Documentation/devicetree/bindings/net/hisilicon-hns-mdio.txt

 Hisilicon MDIO bus controller
 
 Properties:
-- compatible: "hisilicon,mdio","hisilicon,hns-mdio".
+- compatible: can be one of:
+	"hisilicon,hns-mdio"
+	"hisilicon,mdio"
+  "hisilicon,hns-mdio" is recommended to be used for hip05 and later SOCs,
+  while "hisilicon,mdio" is optional for backwards compatibility only on
+  hip04 Soc.
 - reg: The base address of the MDIO bus controller register bank.
 - #address-cells: Must be <1>.
 - #size-cells: Must be <0>.  MDIO addresses have no size component.