fix(zynqmp): query node status to power up APU

If APU is in suspending state and if wakeup request comes then
PMUFW returns error which is not handled at ATF side.

To fix this, get the APU node status before calling wakeup and
return error if found in suspending state.

Here, we can not handle the error code of pm_req_wakeup() from PMUFW
because ATF is already calling pm_client_wakeup() before calling
pm_req_wakeup().

Signed-off-by: Ravi Patel <ravi.patel@xilinx.com>
Signed-off-by: Ronak Jain <ronak.jain@xilinx.com>
Change-Id: I18d47384e46e22ae49e804093ad0641b7a6349e2