stm32: Add support for dual-analog-pad "_C" pins on H7 MCUs.

This commit adds support for the dual-analog-pads on STM32H7 parts.  These
pads/pins are called PA0_C/PA1_C/PC2_C/PC3_C in the datasheet.  They each
have an analog switch that can optionally connect them to their normal pin
(eg PA0).  When the switch is open, the normal and _C pin are independent
pins/pads.

The approach taken in this commit to make these _C pins available to Python
is:
- put them in their own, independent row in the stm32h7_af.csv definition
  file, with only the ADC column defined (they are separate machine.Pin
  entities, and doing it this way keeps make-pins.py pretty clean)
- allow a board to reference these pins in the board's pins.csv file by the
  name PA0_C etc (so a board can alias them, for example)
- these pins (when enabled in pins.csv) now become available like any other
  machine.Pin through both machine.Pin.board and machine.Pin.cpu
- BUT these _C pins have a separate pin type which doesn't have any
  methods, because they don't have any functionality
- these _C pins can be used with machine.ADC to construct the appropriate
  ADC object, either by passing the string as machine.ADC("PA0_C") or by
  passing the object as machine.ADC(machine.Pin.cpu.PA0_C)
- if a board defines both the normal and _C pin (eg both PA0 and PA0_C) in
  pins.csv then it must not define the analog switch to be closed (this is
  a sanity check for the build, because it doesn't make sense to close the
  switch and have two separate pins)

Signed-off-by: Damien George <damien@micropython.org>

ports/stm32/pin.h

@@ -56,6 +56,7 @@ typedef struct {
 } machine_pin_obj_t;
 
 extern const mp_obj_type_t pin_type;
+extern const mp_obj_type_t pin_analog_type;
 extern const mp_obj_type_t pin_af_type;
 
 // Include all of the individual pin objects