ARM: dts: exynos: Use specific compatibles for proper Gscaler limits on Exynos5250 and Exynos5420

Exynos 5250 and 5420 have different hardware rotation limits. However,
currently it uses only one compatible - "exynos5-gsc". Since we have
to distinguish between these two, we add different compatible.

Signed-off-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>