STM32: add CAN support for stm32f3

This enables the CAN support for stm32f3.
The code is almost the same, it just moved to the stm32/common/ folder.
The f3 has only one CAN interface, which resulted in some conflicts, so it had to be separated.
It is tested on f3 hardware.
Can't this whole block just be a include of ```libopencm3/stm32/rcc.h``` ?