clk: samsung: Don't build ARMv8 clock drivers on ARMv7

Currently the Exynos5433 (ARMv8 SoC) clock driver depends on ARCH_EXYNOS
so it is built also on ARMv7. This does not bring any kind of benefit.
There won't be a single kernel image for ARMv7 and ARMv8 SoCs (like
multi_v7 for ARMv7).

Instead build clock drivers only for respective SoC's architecture.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>