clk/exynos5260: add macros and documentation for exynos5260

Add macros which are used as Clock IDs in DT and clock file.
It also adds the documentation for the exynos5260 clocks.

Signed-off-by: Rahul Sharma <Rahul.Sharma@samsung.com>
Signed-off-by: Tomasz Figa <t.figa@samsung.com>