board: rockchip: Add Radxa E20C

The Radxa E20C is an ultra-compact network computer with a RK3528A SoC
that offers a wide range of networking capabilities.

Features tested on a Radxa E20C v1.104:
- SD-card boot
- eMMC boot

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 dcbb335..b992e6c 100644
--- a/doc/board/rockchip/rockchip.rst
+++ b/doc/board/rockchip/rockchip.rst
@@ -99,6 +99,7 @@
 
 * rk3528
      - Generic RK3528 (generic-rk3528)
+     - Radxa E20C (radxa-e20c-rk3528)
 
 * rk3566
      - Anbernic RGxx3 (anbernic-rgxx3-rk3566)