psci: Fix warnings when compiling with W=1

This patch solves the following warnings:
arch/arm/mach-stm32mp/psci.c:

warning: no previous prototype for ‘psci_set_state’ [-Wmissing-prototypes]
warning: no previous prototype for ‘psci_arch_cpu_entry’ [-Wmissing-prototypes]
warning: no previous prototype for ‘psci_features’ [-Wmissing-prototypes]
warning: no previous prototype for ‘psci_version’ [-Wmissing-prototypes]
warning: no previous prototype for ‘psci_affinity_info’ [-Wmissing-prototypes]
warning: no previous prototype for ‘psci_migrate_info_type’ [-Wmissing-prototypes]
warning: no previous prototype for ‘psci_cpu_on’ [-Wmissing-prototypes]
warning: no previous prototype for ‘psci_cpu_off’ [-Wmissing-prototypes]
warning: no previous prototype for ‘psci_system_reset’ [-Wmissing-prototypes]
warning: no previous prototype for ‘psci_system_off’ [-Wmissing-prototypes]

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/arch/arm/include/asm/system.h b/arch/arm/include/asm/system.h
index aed2e3c..a1a5e35 100644
--- a/arch/arm/include/asm/system.h
+++ b/arch/arm/include/asm/system.h
@@ -516,6 +516,21 @@
  */
 void mmu_page_table_flush(unsigned long start, unsigned long stop);
 
+#ifdef CONFIG_ARMV7_PSCI
+void psci_arch_cpu_entry(void);
+u32 psci_version(void);
+s32 psci_features(u32 function_id, u32 psci_fid);
+s32 psci_cpu_off(void);
+s32 psci_cpu_on(u32 function_id, u32 target_cpu, u32 pc,
+		u32 context_id);
+s32 psci_affinity_info(u32 function_id, u32 target_affinity,
+		       u32  lowest_affinity_level);
+u32 psci_migrate_info_type(void);
+void psci_system_off(void);
+void psci_system_reset(void);
+s32 psci_features(u32 function_id, u32 psci_fid);
+#endif
+
 #endif /* __ASSEMBLY__ */
 
 #define arch_align_stack(x) (x)