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: NStephen Boyd <sboyd@codeaurora.org>
Signed-off-by: NKrzysztof Kozlowski <k.kozlowski@samsung.com>
Cc: <stable@vger.kernel.org>
Reviewed-by: NStephen Boyd <sboyd@codeaurora.org>
Signed-off-by: NKukjin Kim <kgene@kernel.org>

arch/arm/mach-exynos/platsmp.c

@@ -126,8 +126,7 @@ static inline void platform_do_lowpower(unsigned int cpu, int *spurious)
  */
 void exynos_cpu_power_down(int cpu)
 {
-	if (cpu == 0 && (of_machine_is_compatible("samsung,exynos5420") ||
-		of_machine_is_compatible("samsung,exynos5800"))) {
+	if (cpu == 0 && (soc_is_exynos5420() || soc_is_exynos5800())) {
 		/*
 		 * Bypass power down for CPU0 during suspend. Check for
 		 * the SYS_PWR_REG value to decide if we are suspending