clk: stm32: add configuration flags for each of the stm32 drivers

Add two configuration flags to be able to not compile all the time
stm32f and stm32h7 drivers when ARCH_STM32 is set.
That help to save some space on those small platforms.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Acked-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>