phy: Add helper function to check phy interface mode

Added helper function that checks phy_mode is RGMII (all variants)
'bool phy_interface_mode_is_rgmii(phy_interface_t mode)'

Changed the following function, to use the above.
'bool phy_interface_is_rgmii(struct phy_device *phydev)'

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Suggested-by: Florian Fainelli <f.fainelli@gmail.com>
Suggested-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>