ARM: dts: exynos: Adjust buck[78] regulators to supported values on Arndale Octa

The datasheet of S2MPS11 PMIC is slightly non-consistent in buck[78]
voltage regulators values.

1. The voltage tables for configuring their registers mention range of
   voltages: 0.750 V to 3.55 V,
2. The constrains in electrical specifications say output voltage range
   to be different (buck7: 1.2 V to 1.5 V, buck8: 1.8 V to 2.1 V).

Adjust the ranges to match the electrical specifications to stay on the
safe side.  Anyway these regulators stay at default value so this should
not have effect.

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