xgene: Don't fail probe, if there is no clk resource for SGMII interfaces

This change fixes following problem

[    1.827940] xgene-enet: probe of 1f210030.ethernet failed with error -2

which leads to a missing ethernet interface (reproducable at least on
Gigabyte MP30-AR0 and APM Mustang systems).

The check for a valid clk resource fails, because DT doesn't provide a
clock for sgenet1. But the driver doesn't use this clk, if the ethernet
port is connected via SGMII. Therefore this patch avoids probing for clk
on SGMII interfaces.

Fixes: 9aea7779

 ("drivers: net: xgene: Fix crash on DT systems")
Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>