cpuidle:clk:Exynos4412: Enable support for clock down when WFI cpuidle state is entered

cpuidle:clk:Exynos4412: Enable support for clock down when WFI cpuidle state is entered

This patch adds support for setting ARM cores' clock frequency down when
entering WFI/WFE based cpuidle state.

On the Trats2 device: performance governor, 1.4 GHz frequency, no extra
load, 4 cores enabled:
Without core clock down feature: 395 mA
With	core clock down feature: 337 mA

Power consumption reduction around 15%

Change-Id: I7bae29b0332a97c7b18ffb79f4b0a5ff3d70b7ce
Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>