arm: Fix compile warning for psci

Commit e71246a23a changes psci_init from a
function returning a void to an int, but does not change the non
CONFIG_ARM_PSCI implementation to return a value, which causes a compile
warning.  Just return 0.

Cc: Ashwin Chaugule <ashwin.chaugule@linaro.org>
Cc: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/arm/include/asm/psci.h

@@ -41,7 +41,7 @@ extern struct smp_operations psci_smp_ops;
 int psci_init(void);
 bool psci_smp_available(void);
 #else
-static inline int psci_init(void) { }
+static inline int psci_init(void) { return 0; }
 static inline bool psci_smp_available(void) { return false; }
 #endif