clk: stm32f4: Update DT bindings documentation

Creation of dt include file for specific stm32f4 clocks.
These specific clocks are not derived from system clock (SYSCLOCK)
We should use index 1 to use these clocks in DT.
e.g. <&rcc 1 CLK_LSI>

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>