Existing code blocks each IPI send request in ipi_mb_notify()
function till pmu clears respective bit in ipi observation
register.

After sending PM_SYSTEM_SHUTDOWN request to PMU, PMU will
restart APU. While PMU is restarting APU, ATF is running out
of OCM, which can cause read/write hang from/to OCM.
There is no need to wait for notification from PMU in case
of SystemShutdown request in ATF, as APU is going to restart.

This patch fixes APU only restart issue.

Signed-off-by: Tejas Patel <tejasp@xilinx.com>
Acked-by: Wendy Liang <wendy.liang@xilinx.com>