phy: marvell: a3700: Access USB3 register indirectly on lane 2

When USB3 is on comphy lane 2 on the Armada 37xx, the registers
have to be accessed indirectly via SATA indirect access.

This is the case of the Turris Mox board from CZ.NIC.

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