Add RPi4 into reference boards list

Added Raspberry pi 4 model B into the reference list for Arm64.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>

content/reference_hardware/arm64.md

@@ -28,4 +28,5 @@ If you currently don't have access to any of the below supported hardware, the
 | Alternative ARM64 reference device | [Renesas R-Car H3 Salvator-X (r8a7795-salvator-x)](http://elinux.org/R-Car/Boards/Salvator-X)   | [Setup guide]( {{< ref "/reference_hardware/rcar-gen3_setup.md" >}} )              |
 | Alternative ARM64 reference device | [Renesas R-Car M3-W Salvator-X (r8a7796-salvator-x)](http://elinux.org/R-Car/Boards/Salvator-X) | [Setup guide]( {{< ref "/reference_hardware/rcar-gen3_setup.md" >}} ) **Untested** |
 |                                    |                                                                                                 |                                                                            |
+| Alternative ARM64 reference device | [Raspberry Pi 4 model B](https://www.raspberrypi.org/products/raspberry-pi-4-model-b/)          | [Setup guide]( {{< ref "/reference_hardware/rpi4_setup.md" >}} )           |