ARM: S3C64XX: Add basic cpuidle driver

Add a very basic cpuidle driver for S3C64xx which merely drives the CPU
into IDLE mode. We could do this with pm_idle but the more modern idiom
is to use cpuidle and the intention is to go further and support STOP
and DEEP-STOP states in conjunction with the pm_domain framework.

The actual state entry code was lifted from Tomasz Figa's work on spica.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>