I noticed secondary CPUs sometimes fail to wake up, and the root
cause is that the sev instruction wakes up slave CPUs before the
preceding the register write is observed by them.

The read-back of the accessed register does not guarantee the order.
In order to ensure the order between the register write and the sev
instruction, a dsb instruction should be executed prior to the sev.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>