ARM: EXYNOS: select THERMAL_OF

We cannot select a symbol that has disabled dependencies, so
we get a warning if we ever enable EXYNOS_THERMAL without
also turning on THERMAL_OF:

warning: (ARCH_EXYNOS) selects EXYNOS_THERMAL which has unmet direct dependencies (THERMAL && (ARCH_EXYNOS || COMPILE_TEST) && THERMAL_OF)

This adds another 'select' in the platform code to avoid that
case. Alternatively, we could decide to not select EXYNOS_THERMAL
here and instead make it a user option.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: f87e6bd3 ("thermal: exynos: Add the dependency of CONFIG_THERMAL_OF instead of CONFIG_OF")
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>