dt-bindings: add documentation for s3c2410 clock controller

Describe the clock controller of s3c2410, s3c2440 and s3c2442.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>