ARM: dts: thermal: exynos: Add documentation for Exynos SoC thermal bindings

Proper description for Exynos4 bindings added to Documentation/devicetree/
bindings. It adds description to use multiple TMU instances, optional voltage
supply node and optional shared register across multiple TMU's.

Acked-by: Jonghwa Lee <jonghwa3.lee@samsung.com>
Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Amit Daniel Kachhap <amit.daniel@samsung.com>
Acked-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>