ARM: EXYNOS: support Exynos4210-bus Devfreq driver on Nuri board

Support varying voltages:
- GPIODVS for Buck2 is removed.
- Voltage ragne for Buck2 is widen.

Support Buck2 regulator for Exynos4210-bus devfreq driver:
- Added device name for buck2 regulator
- Added exynos4210-busfreq platform device fro Nuri board.

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>