ARM: dts: vexpress: Add external expansion bus to DT

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

Add this bus to the VExpress CoreTile device-trees.The bus is described
for a CoreTile occupying site 1.

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>