clk: stm32f7: add dedicated STM32F7 compatible string

Add a dedicated stm32f7 compatible string to use clk_stm32f7
driver with both STM32F4 and STM32F7 SoCs.
It will be needed to manage differences between these 2 SoCs.

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