cpuidle: Fix ARCH_NEEDS_CPU_IDLE_COUPLED dependency warning

Before commit d6f346f2 (cpuidle: improve governor Kconfig options),
the CONFIG_ARCH_NEEDS_CPU_IDLE_COUPLED option didn't depend on
CONFIG_CPU_IDLE but now it has been moved under the CPU_IDLE
menuconfig.

That raises the following warnings:

 warning: (ARCH_OMAP4 && ARCH_TEGRA_2x_SOC) selects ARCH_NEEDS_CPU_IDLE_COUPLED
 which has unmet direct dependencies (CPU_IDLE)
 warning: (ARCH_OMAP4 && ARCH_TEGRA_2x_SOC) selects ARCH_NEEDS_CPU_IDLE_COUPLED
 which has unmet direct dependencies (CPU_IDLE)

because the tegra2 and omap4 Kconfig files select this option
without checking if CPU_IDLE is set.

Fix that by moving ARCH_NEEDS_CPU_IDLE_COUPLED outside of CPU_IDLE.

[rjw: Changelog]
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>