qca4019: Add Meraki Underdog

qca4019: Add Meraki Underdog

Add board file for Meraki Underdog devices: MR20/GR10, MR70/GR60

Board file was extracted from the latest Meraki firmware
for this device (late 2024).

board.json:
```
[
  {
    "names": ["bus=ahb,bmi-chip-id=0,bmi-board-id=22,variant=Meraki-underdog"],
    "data": "board-data/boardData_1_0_IPQ4019_DK04_2G_neg_pwr.bin"
  },
  {
    "names": ["bus=ahb,bmi-chip-id=0,bmi-board-id=23,variant=Meraki-underdog"],
    "data": "board-data/boardData_1_0_IPQ4019_DK04_5G_neg_pwr.bin"
  }
]
```

Signed-off-by: Hal Martin <hal.martin@gmail.com>
Link: https://github.com/openwrt/firmware_qca-wireless/pull/110
Signed-off-by: Robert Marko <robimarko@gmail.com>