arm64: psci: Fix build breakage without PM_SLEEP

Fix build failure of defconfig when PM_SLEEP is disabled (e.g. by
disabling SUSPEND) and CPU_IDLE enabled:

arch/arm64/kernel/psci.c:543:2: error: unknown field ‘cpu_suspend’ specified in initializer
  .cpu_suspend = cpu_psci_cpu_suspend,
  ^
arch/arm64/kernel/psci.c:543:2: warning: initialization from incompatible pointer type [enabled by default]
arch/arm64/kernel/psci.c:543:2: warning: (near initialization for ‘cpu_psci_ops.cpu_prepare’) [enabled by default]
make[1]: *** [arch/arm64/kernel/psci.o] Error 1

The cpu_operations.cpu_suspend field exists only if ARM64_CPU_SUSPEND is
defined, not CPU_IDLE.
Signed-off-by: NKrzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/kernel/psci.c

@@ -540,6 +540,8 @@ const struct cpu_operations cpu_psci_ops = {
 	.name		= "psci",
 #ifdef CONFIG_CPU_IDLE
 	.cpu_init_idle	= cpu_psci_cpu_init_idle,
+#endif
+#ifdef CONFIG_ARM64_CPU_SUSPEND
 	.cpu_suspend	= cpu_psci_cpu_suspend,
 #endif
 #ifdef CONFIG_SMP