Revert "arm64: Juno: Disable CPU idle states for now."

Firmware path that CPUidle uses has now been fixed and
we can go ahead and enable the CPUidle code.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
arm64: Juno: Re-enable the CPUidle support in defconfig.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>