ARM: imx: imx6sx uses imx6q cpuidle code

Building a kernel for imx6sx but without imx6q support results in
this link error because of the missing cpuidle driver:

arch/arm/mach-imx/built-in.o: In function `imx6sx_init_late'::(.init.text+0xc228):
 undefined reference to `imx6q_cpuidle_init'

This patch adds a Makefile entry so we always build support for
the imx6q_cpuidle code when at least one of the 6sx or 6q variants
are enabled.

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