of_mdio: Abstract a general interface for phy connect

Abstract a general interface "of_phy_get_and_connect"
for PHY connect. User will have no bother with getting
"phy-mode" and "phy-handle" any more.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dongpo Li <lidongpo@hisilicon.com>
Reviewed-by: Jiancheng Xue <xuejiancheng@hisilicon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>