thermal: offer Samsung thermal support only when ARCH_EXYNOS is defined

Menu for Samsung thermal support is visible on all Samsung
platforms while thermal drivers are currently available only
for EXYNOS SoCs. Fix it by replacing PLAT_SAMSUNG dependency
with ARCH_EXYNOS one.

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Amit Daniel Kachhap <amit.daniel@samsung.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>