thermal: exynos: Document number of supported trip-points

Document the number of configurable temperature thresholds (for
trip-points in interrupt-driven mode).

Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>