arm64: exynos: Select SOC specific drivers

Recently some Exynos drivers were moved from arm/mach-exynos to
drivers/soc. Building the directory depends on SOC_SAMSUNG Kconfig
option so we want it also on ARMv8 platforms, even though none of the
drivers support ARMv8 SoCs yet.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>