ARM: imx: select HAVE_IMX_SRC when SMP is enabled

With being part of multi-platform support, SMP can be enabled by other
platform even when SOC_IMX6Q is deselected.  It leads to a situation
that arch/arm/mach-imx/platsmp.c is built without HAVE_IMX_SRC selection
which will cause build error.  For example, the following link errors
will be seen when building imx5 with other v7 platforms.

 arch/arm/mach-imx/built-in.o: In function `imx_cpu_die':
 platform-ahci-imx.c:(.text+0x219c): undefined reference to `imx_enable_cpu'
 arch/arm/mach-imx/built-in.o: In function `imx_boot_secondary':
 platform-ahci-imx.c:(.cpuinit.text+0x14): undefined reference to `imx_set_cpu_jump'
 platform-ahci-imx.c:(.cpuinit.text+0x20): undefined reference to `imx_enable_cpu'

Select HAVE_IMX_SRC as long as SMP is enabled to fix the problem.

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