doc: rockchip: update list of Radxa ROCK (Pi) 4 boards

add Radxa ROCK (Pi) 4 variants.

Signed-off-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 a21399c..99376fb 100644
--- a/doc/board/rockchip/rockchip.rst
+++ b/doc/board/rockchip/rockchip.rst
@@ -83,7 +83,10 @@
      - Khadas Edge-V (hadas-edge-v-rk3399)
      - Orange Pi RK3399 (orangepi-rk3399)
      - Pine64 RockPro64 (rockpro64-rk3399)
-     - Radxa ROCK Pi 4 (rock-pi-4-rk3399)
+     - Radxa ROCK 4C+ (rock-4c-plus-rk3399)
+     - Radxa ROCK 4SE (rock-pi-4-rk3399)
+     - Radxa ROCK Pi 4A/B/A+/B+ (rock-pi-4-rk3399)
+     - Radxa ROCK Pi 4C (rock-pi-4c-rk3399)
      - Rockchip Evb-RK3399 (evb_rk3399)
      - Theobroma Systems RK3399-Q7 SoM - Puma (puma_rk3399)