stm32/can: Allow CAN pins to be configured per board.

This patch allows a given board to configure which pins are used for the
CAN peripherals, in a similar way to all the other bus peripherals (I2C,
UART, SPI).  To enable CAN on a board the mpconfigboard.h file should
define (for example):

    #define MICROPY_HW_CAN1_TX (pin_B9)
    #define MICROPY_HW_CAN1_RX (pin_B8)
    #define MICROPY_HW_CAN2_TX (pin_B13)
    #define MICROPY_HW_CAN2_RX (pin_B12)

And the board config file should no longer define MICROPY_HW_ENABLE_CAN.