ARM: dts: stm32: Restore old usart1 clock bindings for stm32f7

As U-boot stm32f clock driver doesn't support new
bindings for auxiliary clocks (clocks = <&rcc 1 ....>),
restore old bindings for usart1 to get console output.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>