board: rockchip: Add Radxa ROCK S0

Radxa ROCK S0 is a single-board computer based on the Rockchip RK3308B
SoC in an ultra-compact form factor. Add a board target for the board.

Features tested on a ROCK S0 v1.2 with 512 MiB RAM and 8 GiB eMMC:
- SD-card boot
- eMMC boot
- Ethernet
- USB gadget
- USB host

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/doc/board/rockchip/rockchip.rst b/doc/board/rockchip/rockchip.rst
index bedc52e..82cbc6e 100644
--- a/doc/board/rockchip/rockchip.rst
+++ b/doc/board/rockchip/rockchip.rst
@@ -54,6 +54,7 @@
      - Amarula Vyasa-RK3288 (vyasa-rk3288)
 * rk3308
      - Radxa ROCK Pi S (rock-pi-s-rk3308)
+     - Radxa ROCK S0 (rock-s0-rk3308)
      - Rockchip Evb-RK3308 (evb-rk3308)
      - Roc-cc-RK3308 (roc-cc-rk3308)
 * rk3326