ARM: DTS: stm32: update rcc compatible for STM32F746

Align the RCC compatible string with the one used by kernel.
It will allow to use the same clock driver for STM32F4
and STM32F7 and to manage the differences between the 2 SoCs

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