arm64: microbench: Add timer_post() to get actual PPI latency

For we get the time duration of (10msec timer + injection latency)
in timer_exec(), we substract the value of 10msec in timer_post()
to get the actual latency.

Signed-off-by: Jingyi Wang <wangjingyi11@huawei.com>
[ Adapt to the modified post() interface. ]
Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>