ARM: dts: add arm/arm64 include symlinks

Allow including of dtsi files in an architecture-independent manner.
Some dtsi files may be shared between architectures and one suggestion
was to have symlinks and let these includes get accessed via a
    #include <arm64/foo.dtsi>
So add the necessary symlinks for arm32.

Suggested-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Olof Johansson <olof@lixom.net>