drivers: marvell: comphy-a3700: support SGMII COMPHY power off

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 BehĂșn <marek.behun@nic.cz>
Change-Id: I55ae0fe3627e7cc0f65c78a00771939d8bf5399f
1 file changed