Building a kernel with the following options,

  CONFIG_SMP=n
  CONFIG_PM=y
  CONFIG_SOC_IMX6SL=y
  CONFIG_SOC_IMX6Q=n

we will see the build error below.

  arch/arm/mach-imx/built-in.o: In function `imx6q_pm_enter':
  platform-spi_imx.c:(.text+0x2648): undefined reference to `v7_cpu_resume'
  make[1]: *** [vmlinux] Error 1

This is because that v7_cpu_resume() implemented in headsmp.S is also
needed by imx6sl build.  Let's build headsmp.S for CONFIG_SOC_IMX6SL as
well.
Reported-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>