remoteproc: zynqmp_r5_remoteproc: use wmb() before kick

Use wmb() to make sure memory operations finished before kicking
another processor. Removing unnecessary and obsolete flush_cache_all().

Signed-off-by: Wendy Liang <jliang@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>