msm: scm: Mark inline asm as volatile

We don't want the compiler to remove these asm statements or
reorder them in any way. Mark them as volatile to be sure.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: David Brown <davidb@codeaurora.org>