psci: put __dead2 attribute after void in plat_psci_ops

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>