net: phy: Add dts support for Motorcomm yt8521 gigabit ethernet phy

Add dts support for Motorcomm yt8521 gigabit ethernet phy.
 Add ytphy_rgmii_clk_delay_config function to support dst config for
 the delay of rgmii clk. This funciont is common for yt8521, yt8531s
 and yt8531.
 This patch has been verified on AM335x platform.
Signed-off-by: Frank Sae <Frank.Sae@motor-comm.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>