arm64: dts: juno/rtsm: re-structure motherboard includes

It is a bit unorthodox to just include a file in the middle of a another
DTS file, it breaks the pattern from other device trees and also makes
it really hard to reference things across the files with phandles.

Restructure the include for the Juno/RTSM motherboards to happen at the
top of the file, reference the target nodes directly, and indent the
motherboard .dtsi files to reflect their actual depth in the hierarchy.

This is a purely syntactic change that result in the same DTB files from
the DTS/DTSI files. This is based on similar patch from Linus Walleij
for ARM Vexpress platforms.

Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>