arm: implement generic PSCI reset call for armv8

Add a psci_system_reset() which calls the SYSTEM_RESET function of
PSCI 0.2 and can be used by boards that support it to implement
reset_cpu().

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/arm/include/asm/system.h

@@ -128,6 +128,8 @@ void hvc_call(struct pt_regs *args);
  */
 void smc_call(struct pt_regs *args);
 
+void __noreturn psci_system_reset(bool smc);
+
 #endif	/* __ASSEMBLY__ */
 
 #else /* CONFIG_ARM64 */