net: phy: smsc: LAN8710/20: add PHY_RST_AFTER_CLK_EN flag

The Microchip/SMSC LAN8710/LAN8720 PHYs need (according to their
datasheet [1]) a continuous REF_CLK when configured to "REF_CLK In Mode".
Therefore set the PHY_RST_AFTER_CLK_EN flag for those PHYs to let the
ETH driver reset them after the REF_CLK is enabled.

[1] http://ww1.microchip.com/downloads/en/DeviceDoc/00002165B.pdf

Signed-off-by: Richard Leitner <richard.leitner@skidata.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>