ARM: imx: select ARM_CPU_SUSPEND only for imx6

ARM: imx: select ARM_CPU_SUSPEND only for imx6

i.MX only needs to select ARM_CPU_SUSPEND manually for the
very specific case that CONFIG_PM_SLEEP is disabled and imx6
is used with CONFIG_PM enabled for runtime PM.

If we are building a kernel only for CPUs that are not using
the cpu_suspend() helper, we otherwise get a harmless
build warning:

warning: (ARCH_MXC && SOC_IMX23 && SOC_IMX28 && ARCH_PXA && MACH_MVEBU_V7 && ARCH_OMAP3 && ARCH_OMAP4 && SOC_OMAP5 && SOC_AM33XX && SOC_DRA7XX && ARCH_EXYNOS3 && ARCH_EXYNOS4 && EXYNOS5420_MCPM &&
EXYNOS_CPU_SUSPEND && ARCH_VEXPRESS_TC2_PM && ARM_BIG_LITTLE_CPUIDLE && ARM_HIGHBANK_CPUIDLE && QCOM_PM) selects ARM_CPU_SUSPEND which has unmet direct dependencies (ARCH_SUSPEND_POSSIBLE)

This moves the option to the SOC_IMX6 option that actually
requires it, in effect reverting commit f36b594f3754 ("ARM:
mach-imx: Select ARM_CPU_SUSPEND at ARCH_MXC level") that was
meant as a cleanup and unintentionally caused this warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>