arm: Add PSCI shutdown function

Using PSCI you can not only reset the system, you can also shut it down!
This patch exposes a function to do exactly that to whatever code wants
to make use of it.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/include/asm/system.h b/arch/arm/include/asm/system.h
index 5166c65..b928bd8 100644
--- a/arch/arm/include/asm/system.h
+++ b/arch/arm/include/asm/system.h
@@ -116,6 +116,7 @@
 void smc_call(struct pt_regs *args);
 
 void __noreturn psci_system_reset(void);
+void __noreturn psci_system_off(void);
 
 #endif	/* __ASSEMBLY__ */