ARM: EXYNOS: Extend cpuidle support to Midas boards

Exynos4412 can only use cpuidle when running under a compatible
bootloader/firmware. The Midas family of boards, including Trats2,
have such a firmware, so allow them to use cpuidle.

Leave the Trats2 check there in order to preserve the ABI.

Signed-off-by: Simon Shields <simon@lineageos.org>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>