arm64: dts: exynos: Add unit address to soc node and move thermal zones on Exynos7

Add @0 unit address to 'soc' node match its 'reg' property and move the
thermal zones out of 'soc' to main root as it this is usually not a
property of a Soc.

This silences DTC warnings:

    Warning (unit_address_vs_reg): /soc: node has a reg or ranges property, but no unit name
    Warning (simple_bus_reg): /soc/thermal-zones: missing or empty reg/ranges property

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