ARM: dts: exynos: Add Mali/GPU node on Exynos5420 and enable it on Odroid XU3/4

Add device tree node for Mali GPU on Exynos 542x SoC family.  GPU is
disabled by default, and is enabled for each board after the regulator
is defined. Tested on Odroid XU4.

Signed-off-by: Marian Mihailescu <mihailescu2m@gmail.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>