ARM: imx6: fix SMP compilation again

My earlier patch 1fc593fe

 ("ARM: imx: build i.MX6 functions
only when needed") fixed a problem with building an i.MX5 kernel,
but now the problem has returned for the case where we allow
ARMv6K SMP builds in multiplatform. With CONFIG_CPU_V7 disabled,
but i.MX3 and SMP enabled, we get this build error:

arch/arm/mach-imx/built-in.o: In function `v7_secondary_startup':
:(.text+0x5124): undefined reference to `v7_invalidate_l1'

This puts the code inside of an "ifdef CONFIG_SOC_IMX6" to hopefully
do the right thing in all configurations.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>