clk: stm32f7: remove clock_get()

All drivers which was using clock_get() are now using
clk_get_rate() from clock framework, now it's safe to
remove clock_get().

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