ARM: qcom: 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
response buffers instead, moving the responsibility of flushing any other
cached buffer (being passed to the secure world) to callers.

Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Kumar Gala <galak@codeaurora.org>