phy: marvell: mux: Support nontrivial node order in selector register

Currently comphy_mux supports only trivial order of nodes in pin
selector register, that is lane N on position N*bitcount.

Add support for nontrivial order, with map stored in device tree
property mux-lane-order.

This is needed for Armada 37xx.

As far as I know, there is no driver for Armada 37xx comphy in the
kernel. When such a driver comes, this will need to be rewritten to
support the device tree bindings from the kernel.

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