net: Add MDIO bus driver for the Hisilicon FEMAC

This patch adds a separate driver for the MDIO interface of the
Hisilicon Fast Ethernet MAC.

Signed-off-by: Dongpo Li <lidongpo@hisilicon.com>
Reviewed-by: Jiancheng Xue <xuejiancheng@hisilicon.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>