arm64: optimize smp_kick_all_cpus

gic_kick_secondary_cpus can directly return to the caller of
smp_kick_all_cpus.  We do not have to use x29 register here.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: York Sun <york.sun@nxp.com>