clk: exynos5433: Add documentation for the audio block parent clocks

Audio block requires access to two parent clocks: audio PLL and oscillator,
so add this information to device tree bindings documentation.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>