msm: npu: Use write memory barrier at the end of npu_mem_write

When sending IPC command to firmware, command will be written
into SSTCM using npu_mem_write, then trigger the interrupt.
A write memory barrier is required at the end of npu_mem_write
to make sure that the interrupt won't be triggered until command
has been completely written into SSTCM.

Change-Id: I9cd904929d52197ae9bf0429075c1d45ccd5b9f0
Signed-off-by: Da Hoon Pyun <dpyun@codeaurora.org>