ARM: exynos: use private samsung_cpu_id copy

The only part of plat-samsung that is shared with arch-exynos
is the CPU identification code.

Having a separate exynos_cpu_id variable makes the two completely
independent and is actually a bit less code in total.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20200806182059.2431-14-krzk@kernel.org


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