arm64: dts: marvell: use aliases for SPI busses on Armada 7K/8K

We are currently using the cell-index DT property to assign SPI bus
numbers. This property is specific to the spi-orion driver, and
requires each SPI controller to have a unique ID defined in the Device
Tree.

As we are about to merge armada-cp110-master.dtsi and
armada-cp110-slave.dtsi into a single file, those cell-index
properties that differ between the master CP110 and the slave CP110
are a difference that would have to be handled.

In order to avoid this, we switch to using the "aliases" DT node to
assign a unique number to each SPI controller. This is more generic,
and directly handled by the SPI core.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>