arm64: Juno: Split juno.dts into juno-base.dtsi and juno.dts.

Prepare the device tree for adding more boards based on Juno r0.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Acked-by: Jon Medhurst <tixy@linaro.org>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>