stm32/pyb_can: Add ability to calculate CAN bit timing from baudrate.

Calculate the bit timing from baudrate if provided, allowing sample point
override.  This makes it a lot easier to make CAN work between different
MCUs with different clocks, prescalers etc.

Tested on F4, F7 and H7 Y/V variants.

docs/library/pyb.CAN.rst

@@ -49,7 +49,7 @@ Class Methods
 Methods
 -------
 
-.. method:: CAN.init(mode, extframe=False, prescaler=100, *, sjw=1, bs1=6, bs2=8, auto_restart=False)
+.. method:: CAN.init(mode, extframe=False, prescaler=100, *, sjw=1, bs1=6, bs2=8, auto_restart=False, baudrate=0, sample_point=75)
 
    Initialise the CAN bus with the given parameters:
 
@@ -67,6 +67,11 @@ Methods
      - *auto_restart* sets whether the controller will automatically try and restart
        communications after entering the bus-off state; if this is disabled then
        :meth:`~CAN.restart()` can be used to leave the bus-off state
+     - *baudrate* if a baudrate other than 0 is provided, this function will try to automatically
+       calculate a CAN bit-timing (overriding *prescaler*, *bs1* and *bs2*) that satisfies both
+       the baudrate and the desired *sample_point*.
+     - *sample_point* given in a percentage of the bit time, the *sample_point* specifies the position
+       of the last bit sample with respect to the whole bit time. The default *sample_point* is 75%.
 
    The time quanta tq is the basic unit of time for the CAN bus.  tq is the CAN
    prescaler value divided by PCLK1 (the frequency of internal peripheral bus 1);