ARM: imx6: build headsmp.o only on CONFIG_SMP

With v7_cpu_resume() being moved out of headsmp.S, all the remaining
code in the file is only needed by CONFIG_SMP build.  So we can control
the build of headsmp.o with only obj-$(CONFIG_SMP) now.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>