ARM: dts: stm32: re-add CAN support on stm32f746

The revert commit 36a6418b ("Revert "ARM: dts: stm32: add CAN support
on stm32f746"") prevented parsing errors due to the lack of CAN3 binding.

Now that the binding definition for CAN3 is available in the mainline
thanks to commit 8f3ef556 ("dt-bindings: mfd: stm32f7: Add binding
definition for CAN3"), we can re-add the CAN support and make the driver
usable again.

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