ARM: dts: stm32: use RCC macro for CRC node on stm32f746

The patch replaces the number 12 with the appropriate numerical constant
already defined in the file stm32f7-rcc.h.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>