doc: Unbricking instruction for SparX-5i eval board

For...you know...other people.

board/aarch64/README.md

@@ -12,3 +12,48 @@ FIT options:
 
     make board-enable-sparx-fit
 
+### Unbricking
+
+1. Don't load a corrupt bootloader to begin with
+2. Done
+
+If, for some reason, you didn't manage to follow step 1, you can use
+the `FLASH_PROG`(`J4`) connector on the board to access the SPI flash
+directly.
+
+Using a [Bus Blaster][BB3] in combination with [dangerspi][dangerspi],
+you can load a new bootloader. The schematic below details how to
+connect the Bus Blaster to the board, but the concept should be
+portable to any debugger/device based around an FTDI2232 chip.
+
+```
+Bus Blaster:         FLASH_PROG (J4):
+       .---.              .---.
+   VTG >o o| VTG      SCK >o o| GND
+  TRST |o o| GND     MISO |o o| #RESET_FLASH
+   TDI |o o| GND           o o| 3V3
+   TMS |o o| GND      #CS |o o| #SYSRESET
+   TCK  o o| GND     MOSI |o o| GND
+  RTCK  o o| GND          '---'
+   TDO |o o| GND
+ TSRST |o o| GND
+ DBGRQ |o o| GND
+DBGACK |o o| GND
+       '---'
+```
+
+| Bus Blaster | FLASH_PROG     |
+|-------------|----------------|
+| `VTG`       | `3V3`          |
+| `GND`       | `GND`          |
+|             |                |
+| `VTG`       | `#RESET_FLASH` |
+| `GND`       | `#SYSRESET`    |
+|             |                |
+| `TDI`       | `MOSI`         |
+| `TMS`       | `#CS`          |
+| `TCK`       | `SCK`          |
+| `TDO`       | `MISO`         |
+
+[BB3]: http://dangerousprototypes.com/docs/Bus_Blaster#Bus_Blaster_v3
+[dangerspi]: https://github.com/wkz/dangerspi