phy: marvell: a3700: Save/restore selector reg in SGMII init

In SGMII initialization PIN_PIPE_SEL has to be zero when resetting
the PHY. Since comphy_mux already set the selector register to
correct values, we have to store it's value before setting it to 0
and restore it after SGMII init.

Signed-off-by: Marek Behun <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>