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: I41a8cf058351088fd1ce2b33117084d773a6836f
Signed-off-by: Jilai Wang <jilaiw@codeaurora.org>