dts: arm: st: g4/u5: add missing fdcan clocks

G4 and U5 series missed clock information in DT. Driver likely worked
because it was using HAL helpers, bypassing the purpose of DT and clock
control drivers. The clocks property is now required in the binding
file.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>