ARM: dts: Update MFC documentation for clock entries

Added clock entry definitions to MFC bindings document.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>