clk: stm32f7: add clock .get_rate() callback

Add clock framework .get_rate callback.
This step will allow to convert all drivers which was using
proprietary clock_get() to use clock framework .get_rate().

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Acked-by: Vikas MANOCHA <vikas.manocha@st.com>