firmware: zynqmp: fix write to an uninitialised pointer in ipi_req()

When a caller is not interested in the returned message, the ret_payload
pointer is set to NULL in the u-boot-sources. In this case, under EL3, the
memory from address 0x0 would be overwritten by ipi_req() with the returned
IPI message, damaging the original data under this address. The patch, in
case ret_payload is NULL, assigns the pointer to the array holding the IPI
message being sent.

Signed-off-by: Adrian Fiergolski <adrian.fiergolski@fastree3d.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Adrian Fiergolski <Adrian.Fiergolski@fastree3d.com>
Link: https://lore.kernel.org/r/3178ff7651948270b714daa4adad48b94eaca9ba.1634309856.git.michal.simek@xilinx.com