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Add support for powering off the SGMII COMPHY (on lanes 0 and 1).
This is needed sometimes on Turris Mox when using KEXEC.

There is also another benefit of a little energy saving when the given
network interface is down.
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Change-Id: I55ae0fe3627e7cc0f65c78a00771939d8bf5399f