net: phy: dp83867: fix hfs boot in rgmii mode

The commit ef87f7da ("net: phy: dp83867: move dt parsing to probe")
causes regression on TI dra71x-evm and dra72x-evm, where DP83867 PHY is
used in "rgmii-id" mode - the networking stops working.
Unfortunately, it's not enough to just move DT parsing code to .probe() as
it depends on phydev->interface value, which is set to correct value abter
the .probe() is completed and before calling .config_init(). So, RGMII
configuration can't be loaded from DT.

To fix and issue
- move RGMII validation code to .config_init()
- parse RGMII parameters in dp83867_of_init(), but consider them as
optional.

Fixes: ef87f7da

 ("net: phy: dp83867: move dt parsing to probe")
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>