ARM: msm: fix compilation flags for MSM_SCM

CONFIG_MSM_SCM uses the smc instruction, which with some
toolchains requires a ".arch_extension" directive.

Cc: David Brown <davidb@codeaurora.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: David Brown <davidb@codeaurora.org>