xgene: Always get clk source, but ignore if it's missing for SGMII ports

Even the driver doesn't do anything with the clk source for SGMII
ports it needs to be enabled by doing a devm_clk_get(), if there is
a clk source in DT.

Fixes: 0db01097 ('xgene: Don't fail probe, if there is no clk resource for SGMII interfaces')
Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Tested-by: Laura Abbott <labbott@redhat.com>
Acked-by: Iyappan Subramanian <isubramanian@apm.com>
Tested-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>