doc: rockchip: Add phyCORE-RK3288 RDK to board list

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
Acked-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>

doc/README.rockchip

@@ -36,13 +36,14 @@ You will need:
 Building
 ========
 
-At present eight RK3288 boards are supported:
+At present nine RK3288 boards are supported:
 
    - EVB RK3288 - use evb-rk3288 configuration
    - Fennec RK3288 - use fennec-rk3288 configuration
    - Firefly RK3288 - use firefly-rk3288 configuration
    - Hisense Chromebook - use chromebook_jerry configuration
    - MiQi RK3288 - use miqi-rk3288 configuration
+   - phyCORE-RK3288 RDK - use phycore-rk3288 configuration
    - PopMetal RK3288 - use popmetal-rk3288 configuration
    - Radxa Rock 2 - use rock2 configuration
    - Tinker RK3288 - use tinker-rk3288 configuration