net: phy: Add phy loopback support in net phy framework

This patch add set_loopback in phy_driver, which is used by MAC
driver to enable or disable phy loopback. it also add a generic
genphy_loopback function, which use BMCR loopback bit to enable
or disable loopback.

Signed-off-by: Lin Yun Sheng <linyunsheng@huawei.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>