cpuidle: big.LITTLE: enable driver only on Peach-Pit/Pi Chromebooks

This driver always worked properly only on the Exynos 5420/5800 based
Chromebooks (Peach-Pit/Pi), so change the required compatible string to
the 'google,peach', to avoid enabling it on the other Exynos 542x/5800
boards, which hangs in such case. The main difference between Peach-Pit/Pi
and other Exynos 542x/5800 boards is the firmware - Peach platform doesn't
use secure firmware at all.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>