psci: put __dead2 attribute after void in plat_psci_ops

These warnings were issued by sparse:
plat/st/stm32mp1/stm32mp1_pm.c:365:36:
 warning: incorrect type in initializer (different modifiers)
    expected void ( *[noreturn] pwr_domain_pwr_down_wfi )( ... )
    got void ( [noreturn] *<noident> )( ... )
plat/st/stm32mp1/stm32mp1_pm.c:366:23:
 warning: incorrect type in initializer (different modifiers)
    expected void ( *[noreturn] system_off )( ... )
    got void ( [noreturn] *<noident> )( ... )
plat/st/stm32mp1/stm32mp1_pm.c:367:25:
 warning: incorrect type in initializer (different modifiers)
    expected void ( *[noreturn] system_reset )( ... )
    got void ( [noreturn] *<noident> )( ... )

This cannot be changed the other way in all platforms pm drivers
or else there is a compilation error:
plat/st/stm32mp1/stm32mp1_pm.c:234:1: error: attributes should be specified
 before the declarator in a function definition
Signed-off-by: Yann Gautier <yann.gautier@st.com>

include/lib/psci/psci.h

@@ -302,10 +302,10 @@ typedef struct plat_psci_ops {
 	void (*pwr_domain_on_finish)(const psci_power_state_t *target_state);
 	void (*pwr_domain_suspend_finish)(
 				const psci_power_state_t *target_state);
-	void (*pwr_domain_pwr_down_wfi)(
-				const psci_power_state_t *target_state) __dead2;
-	void (*system_off)(void) __dead2;
-	void (*system_reset)(void) __dead2;
+	void __dead2 (*pwr_domain_pwr_down_wfi)(
+				const psci_power_state_t *target_state);
+	void __dead2 (*system_off)(void);
+	void __dead2 (*system_reset)(void);
 	int (*validate_power_state)(unsigned int power_state,
 				    psci_power_state_t *req_state);
 	int (*validate_ns_entrypoint)(uintptr_t ns_entrypoint);