drivers: clk: st: Simplify clock binding of STiH4xx platforms

This patch reworks the clock binding to avoid too much detail in DT.
Now we have only compatible string per type of clock
(remark from Rob https://lkml.org/lkml/2016/5/25/492

)

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Acked-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>