ARM: exynos: stop selecting PLAT_SAMSUNG

Now that no code in arch/arm is shared between mach-exynos and the
others, make the split formal.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20200806182059.2431-15-krzk@kernel.org


Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>