msm: scm: Flush the command buffer only instead of the entire cache

scm_call flushes the entire cache before calling into the
secure world. This is both a performance penalty as well
as insufficient on SMP systems where the CPUs possess a
write-back L1 cache. Flush only the command and reponse
buffers instead, moving the responsibility of flushing any
other cached buffer (being passed to the secure world) to
callers.

Change-Id: Ic5dabf04a51aa2b190a841445e86d1c3f330f27c
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>