Follow the pattern we have for other peripherals in that if the driver
class (CAN) is enabled than enable the driver for that class
(CAN_STM32). Also have the STM32 CAN driver depend on being on a STM32
SoC.
Remove setting of CONFIG_CAN_STM32 in any .conf files as it will get set
of CONFIG_CAN is set/enabled.
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>