net: phy: at803x: add downshift support

The AR8031 and AR8035 support the link speed downshift. Add driver
support for it. One peculiarity of these PHYs is that it needs a
software reset after changing the setting, thus add the .soft_reset()
op and do a phy_init_hw() if necessary.

This was tested on a custom board with the AR8031.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>