cpuidle: big.LITTLE: add Exynos5800 compatible string

Exynos 5800 is big.LITTLE SoC compatible with the 5420.  Add the
compatible string so this driver works on the 5800.

Tested on exynos5800-peach-pi (aka Samsung Chromebook2)

Signed-off-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>