px4_micro_hal:Define PX4_NUMBER_I2C_BUSES based on the arch/chip
px4_micro_hal:Define PX4_NUMBER_I2C_BUSES based on the arch/chip

  PX4_NUMBER_I2C_BUSES number of busses that the HW can support
  not all of them my be usesd. I.E. The STM32F427 has 3 I2C
  busses but only I2C1 and I2C3 are used.

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