rockchip: Add support for Radxa ROCK 4C+

The Radxa ROCK 4C+[1] is a single board computer with dual HDMI using
the Rockchip RK3399-T.

Hardware
--------

- Dual-core Cortex-A72 and quad-core Cortex-A53 CPU
- Mali-T860MP4 GPU
- LPDDR4 4GB RAM
- eMMC connector
- microSD card slot
- Wi-Fi 5 (not supported)
- Gigabit Ethernet with PoE support (additional PoE HAT required)
- USB 3.0 Type-A OTG port
- USB 3.0 Type-A HOST port
- 2x USB 2.0 Type-A HOST ports
- USB Type-C power port (5V only)
- 40 Pin GPIO header

[1] https://radxa.com/products/rock4/4cp

Installation
------------
Uncompress the OpenWrt sysupgrade and write it to the micro SD card or
internal eMMC using dd.

Author: coolsnowwolf

package/boot/uboot-rockchip/Makefile

@@ -152,6 +152,13 @@ define U-Boot/rock-4se-rk3399
     radxa_rock-4se
 endef
 
+define U-Boot/rock-4c-plus-rk3399
+  $(U-Boot/rk3399/Default)
+  NAME:=ROCK 4C+
+  BUILD_DEVICES:= \
+    radxa_rock-4c-plus
+endef
+
 define U-Boot/rock-pi-4-rk3399
   $(U-Boot/rk3399/Default)
   NAME:=Rock Pi 4
@@ -523,6 +530,7 @@ UBOOT_TARGETS := \
   nanopc-t4-rk3399 \
   nanopi-r4s-rk3399 \
   nanopi-r4se-rk3399 \
+  rock-4c-plus-rk3399 \
   rock-pi-4-rk3399 \
   rock-4se-rk3399 \
   rockpro64-rk3399 \