arm64: dts: exynos: Modify the voltage range for BUCK2 for exynos7

Change the BUCK2 (vdd_atlas) voltage range to '500 - 1200mv' since
CPU DVFS requires it.

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>