arm64: dts: juno: remove dtsi nesting inside tree structure

Currently juno-clock.dtsi and juno-base.dtsi are nested badly inside
the device tree structure. It's generally good practice to ensure that
individual dtsi stand by themselves at the top of the file.

This patch removes the nesting of the above mentioned dtsi files and
makes them independent.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>