ARM: dts: exynos: Clarify comment explaining purpose of Odroid XU3 DTSI

There are two common DTSI files for Exynos5422 Odroid XU3 family of
boards.  One is shared between all of them (XU3, XU3-Lite, XU4 and HC1)
and the second skips HC1.  Document this in the files.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>