board: rockchip: Add Radxa ROCK 3B

The Radxa ROCK 3B is a single-board computer based on the Pico-ITX form
factor (100mm x 75mm). Two versions of the ROCK 3B exists, a community
version based on the RK3568 SoC and an industrial version based on the
RK3568J SoC.

Features tested on ROCK 3B 8GB v1.51 (both variants):
- SD-card boot
- eMMC boot
- SPI Flash boot
- Ethernet
- PCIe/NVMe
- USB gadget
- USB host

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Tested-by: FUKAUMI Naoki <naoki@radxa.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/doc/board/rockchip/rockchip.rst b/doc/board/rockchip/rockchip.rst
index a78d6db..7115cfb 100644
--- a/doc/board/rockchip/rockchip.rst
+++ b/doc/board/rockchip/rockchip.rst
@@ -117,7 +117,8 @@
      - Generic RK3566/RK3568 (generic-rk3568)
      - Hardkernel ODROID-M1 (odroid-m1-rk3568)
      - Radxa E25 Carrier Board (radxa-e25-rk3568)
-     - Radxa ROCK 3 Model A (rock-3a-rk3568)
+     - Radxa ROCK 3A (rock-3a-rk3568)
+     - Radxa ROCK 3B (rock-3b-rk3568)
 
 * rk3588
      - ArmSoM Sige7 (sige7-rk3588)