dt-bindings: add documentation for Exynos3250 clock controller

The Exynos3250 clocks are statically listed and registered using the
Samsung specific common clock helper functions. Both device tree based
clock lookup and clkdev based clock lookups are supported.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Mike Turquette <mturquette@linaro.org>
Cc: Kukjin Kim <kgene.kim@samsung.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>