stm32: Support pins with multiple AFs for the same peripheral.

For example, on WB55, pin PD3 can be SPI2_SCK on AF3 and SPI2_MISO on AF5.

PortD,PD3,,,,SPI2_SCK,,SPI2_MISO,,,,,QUADSPI_BK1_NCS,,,,,EVENTOUT,

When we configure a default pin in mpconfigboard.h, it is unable (via pin_find_af and the generated code from make-pins.py) to resolve this because the AF_FN_SPI enum assumes that a given peripheral will always use the same AF number on all pins.

// This will fail, and make D3 be SPI2_SCK instead.
#define MICROPY_HW_SPI2_MISO        (pin_D3)

Here are some other AF CSV files and peripherals where different AFs share the same peripheral. Other than the WB55 SPI2 example I didn't see any that are accessible from Python. (i.e. the I2C4 are conflicting with I2C_SMBA).

boards/stm32f091_af.csv
    TIM15, USART7
boards/stm32f405_af.csv
    OTG
boards/stm32f429_af.csv
    LCD, OTG
boards/stm32f439_af.csv
    LCD, OTG
boards/stm32f722_af.csv
    OTG
boards/stm32f746_af.csv
    LCD, OTG
boards/stm32f767_af.csv
    OTG, DFSDM1, LCD, I2C4
boards/stm32h743_af.csv
    DFSDM, TIM8, SDMMC1, LCD, SAI1, TIM1, I2C4, OTG, SAI4
boards/stm32h7b3_af.csv
    OCTOSPIM, DFSDM1, PSSI, FMC, TIM8, LTDC, SDMMC1, SAI1, TIM1, I2C4
boards/stm32l072_af.csv
    TIM2, LPUART1
boards/stm32l432_af.csv
    TIM1, TIM2, COMP2, COMP1
boards/stm32l452_af.csv
    TIM1, TIM2, COMP2, COMP1
boards/stm32l476_af.csv
    TIM5, TIM2, TIM1, TIM8
boards/stm32l496_af.csv
    TIM2, SPI2, TIM8, TIM5, TIM1, QUADSPI
boards/stm32wb55_af.csv
    TIM2, TIM1, SAI1, SPI2

Start with pin_find_af, the generated code from tools/make-pins.py, and the AF_FN_SPI enum handling described in the issue. Review boards/stm32wb55_af.csv and mpconfigboard.h, then compare the other listed AF CSV files for similar conflicts. Done means a peripheral can resolve the correct alternate function when one pin exposes multiple AFs, including the SPI2 example.