xen/arm64: Avoid sending SGI when kicking secondary cpus with spin_table

xen/arm64: Avoid sending SGI when kicking secondary cpus with spin_table

On arm64, either firmware or xen's smp_up_cpu gate uses WFE on secondary
cpus to stand-by when booting. Thus, using SEV is enough for the boot
cpu to kick other secondaries. Further more, the current implementation
of cpu_up_send_sgi would pass a NULL cpumask pointer to send_SGI, which
then lead a data fault on GICv3 send_SGI implementation.

Signed-off-by: Chen Baozi <baozich@gmail.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>