Panfrost driver provides a devfreq driver for the Mali GPU and allows to
scale GPU core frequency. Enable support for devfreq thermal integration
to enable cooling of GPU thermal zone by reducing GPU core frequency.

This fixes following warning during boot on Exynos5422-based Odroid XU4:

panfrost 11800000.gpu: [drm:panfrost_devfreq_init] Failed to register cooling device
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>