net: ethernet: nb8800: Handle all 4 RGMII modes identically

Before commit bf8f6952 ("Add blurb about RGMII") it was unclear
whose responsibility it was to insert the required clock skew, and
in hindsight, some PHY drivers got it wrong. The solution forward
is to introduce a new property, explicitly requiring skew from the
node to which it is attached. In the interim, this driver will handle
all 4 RGMII modes identically (no skew).

Fixes: 52dfc830 ("net: ethernet: add driver for Aurora VLSI NB8800 Ethernet controller")
Signed-off-by: Marc Gonzalez <marc_gonzalez@sigmadesigns.com>
Signed-off-by: David S. Miller <davem@davemloft.net>