thermal: exynos: List vtmu-supply as optional property in DT binding

The Exynos Thermal Management Unit binding says that the vtmu-supply
is optional but is listed in the required properties section. Add an
optional properties section and move the regulator property there.

Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Andi Shyti <andi.shyti@samsung.com>
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>