clk: stm32f7: Add stm32f7 clock DT bindings for STM32F746 boards

This patch introduces the stm32f7 clock DT bindings.

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