clk: stm32f7: rename clk_stm32f7.c to clk_stm32f.c

Now that clk_stm32f7.c manages clocks for both STM32F4 and F7 SoCs
rename it to a more generic clk_stm32f.c

Fix also some checkpatch errors/warnings.

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