px4fmu-v2: Add detection for Pixhack V3 (#10231)

The Simple HW detection was detecting the
  Pixhack V3 as a mini.

  This commit adds further discrimination, based on
  the fact the Pixhack V3 left VDD_5V_PERIPH_EN
  floating. Where a mini ,V2, V3 HW has it connected
  to the active low enable of the VDD_5V_PERIPH and
  VDD_5V_HIGHPOWER via a 10K pull down.

  The detection enables the 40K pull up and samples
  the pin. This reads back as a logical one on the
  Pixhack V3, and a logical zero on the Mini.

  Since the float is applied some 3.3 Ms post pin setting
  at reset the 10 us should leave the 5 Volt supplies in
  the on state because of the 10K pull down.
  This is assuming typical 40K pullup 10K pull down
  This results in typical 0.66V (.875V max) on the /EN pin
  which should not be detected as a high on the TPS2041.