thermal: exynos: Add support for instance based register/unregister

This code modifies the thermal driver to have multiple thermal zone
support by replacing the global thermal zone variable with device data
member of thermal_zone_device.

Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Acked-by: Jonghwa Lee <jonghwa3.lee@samsung.com>
Acked-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Amit Daniel Kachhap <amit.daniel@samsung.com>
Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>