arm64: dts: juno: Add external expansion bus to DT

The Juno development platform has an external expansion bus which can
be used for additional hardware (e.g. LogicTile Express daughterboards).

Add this bus to the Juno base device-tree.

Acked-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Brian Starkey <brian.starkey@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>