net/phy/vitesse: Configure RGMII skew on VSC8601, if needed

With RGMII, we need a 1.5 to 2ns skew between clock and data lines. The
VSC8601 can handle this internally. While the VSC8601 can set more
fine-grained delays, the standard skew settings work out of the box.
The same heuristic is used to determine when this skew should be enabled
as in vsc824x_config_init().

Tested on custom board with AM3352 SOC and VSC801 PHY.

Signed-off-by: Alexandru Gagniuc <alex.g@adaptrum.com>
Signed-off-by: David S. Miller <davem@davemloft.net>