arm64: dts: exynos: Add GPU/Mali T760 node to Exynos7

Add nodes for GPU (Mali T760) to Exynos7.  Current support for Exynos7
misses a lot, including proper clocks, power domains, frequency and
voltage scaling and cooling.  However this still can provide basic GPU
description.  Not tested on HW.

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