ARM: EXYNOS: Disable cpuidle for exynos5440

There is no point to register the cpuidle driver for the 5440 as it has only
one WFI state which is the default idle function when the cpuidle driver is
disabled.

By disabling cpuidle we prevent to enter to the governor computation for
nothing, thus saving a lot of processing time.

The only drawback is the statistic via sysfs on this state which is lost but
it is meaningless and it could be retrieved from the ftrace easily.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Acked-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>