ARM: imx6: fix missing <soc/imx/cpuidle.h> in cpuidle-imx6q.c

The <soc/imx/cpuidle.h> file has declarations of two funcitons
exported from cpuidle-imx6q.c but it is not included. Fix the
following warnings by adding the include:

arch/arm/mach-imx/cpuidle-imx6q.c:71:6: warning: symbol 'imx6q_cpuidle_fec_irqs_used' was not declared. Should it be static?
arch/arm/mach-imx/cpuidle-imx6q.c:76:6: warning: symbol 'imx6q_cpuidle_fec_irqs_unused' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>