ARM: imx: fix build error with !CONFIG_SMP

ARM: imx: fix build error with !CONFIG_SMP

Commit 68b2532 (ARM: imx: select HAVE_IMX_SRC when SMP is enabled)
introduces a build error with imx_v6_v7_defconfig when CONFIG_SMP is
deselected.

  LINK    vmlinux
  LD      vmlinux.o
  MODPOST vmlinux.o
  GEN     .version
  CHK     include/generated/compile.h
  UPD     include/generated/compile.h
  CC      init/version.o
  LD      init/built-in.o
arch/arm/mach-imx/built-in.o: In function `imx6q_restart':
platform-ahci-imx.c:(.text+0x448c): undefined reference to `imx_src_prepare_restart'
arch/arm/mach-imx/built-in.o: In function `imx6q_pm_enter':
platform-ahci-imx.c:(.text+0x4544): undefined reference to `imx_set_cpu_jump'
arch/arm/mach-imx/built-in.o: In function `imx6q_init_irq':
platform-ahci-imx.c:(.init.text+0xbef0): undefined reference to `imx_src_init'
make[1]: *** [vmlinux] Error 1

While the commit adds 'def_bool y if SMP' for HAVE_IMX_SRC, it should
not remove 'select HAVE_IMX_SRC' from SOC_IMX6Q, as the IMX6Q UP build
also needs HAVE_IMX_SRC.  Add the HAVE_IMX_SRC select back for SOC_IMX6Q
to fix above build error.

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