ARM: psci-smp is built only for SMP

The PSCI SMP implementation is built only when both CONFIG_SMP and CONFIG_ARM_PSCI are set, so a configuration that has the latter but not the former can get a link error when it tries to call psci_smp_available().

arch/arm/mach-tegra/built-in.o: In function `tegra114_cpuidle_init':
cpuidle-tegra114.c:(.init.text+0x52a): undefined reference to `psci_smp_available'

This corrects the #ifdef in the psci.h header file to match the Makefile conditional we have for building that function.

index 68ee3ce17b82..b4c6d99364f1 100644
@@ -16,7 +16,7 @@
 
 extern struct smp_operations psci_smp_ops;
 
-#ifdef CONFIG_ARM_PSCI
+#if defined(CONFIG_SMP) && defined(CONFIG_ARM_PSCI)
 bool psci_smp_available(void);
 #else
 static inline bool psci_smp_available(void) { return false; }