ARM: exynos_defconfig: Enable devfreq thermal integration

ARM: exynos_defconfig: Enable devfreq thermal integration

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>