arm: mvebu: turris_mox: Set "sfp" label in eth1 DT node when only Mox SFP is detected

When Mox SFP module is connected after Topaz or Peridot module then port DT
node already contains "sfp" label. But Mox SFP module can be connected also
without Topaz or Peridot module in which case it is connected directly into
he eth1 DT node, which is without any label. So add "sfp" label into eth1
DT node in this case.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>