ARM: dts: vexpress: Restructure 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 Versatile Express 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.

Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Mali DP Maintainers <malidp@foss.arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>