ARM: EXYNOS: Don't use LDREX and STREX after disabling cache coherency

During CPU shutdown the exynos_cpu_power_down() is called after
disabling cache coherency and it uses LDREX and STREX instructions (by
calling of_machine_is_compatible() -> kobject_get() -> kref_get()).

The LDREX and STREX should not be used after disabling the cache
coherency so just use soc_is_exynos().

Fixes: adc548d7 ("ARM: EXYNOS: Use MCPM call-backs to support S2R
on exynos5420")

Reported-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Cc: <stable@vger.kernel.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Kukjin Kim <kgene@kernel.org>