Add bdf for Xiaomi Pad 5 Pro, which running mainline kernel.

* description for what hardware this is:

  - it is a SM8250 based board
  - it use QCA6390 as wlan module

* origin of the board file (did you create it yourself or where you
  downloaded)

  - taken from stock firmware image

* ids to be used with the board file (ATH10K_BD_IE_BOARD_NAME in ath10k)

  - QCA6390 hw2.0

    + bus=pci,qmi-chip-id=0,qmi-board-id=255,variant=Xiaomi_Pad_5Pro
      sha256sum:
	  a5c99867d398b4e8773d106100e8575e1b3c29b8e18921dae50029b1253eac06

* attach the actual board file (board.bin)

  - The name of the files are equal to the id string in the board-2.bin
    (minus the ".bin")