rockchip: add support for Radxa ROCK 3A

Radxa ROCK 3A is a credit card sized SBC with rich interfaces[1] using
the Rockchip RK3328 SoC.

Hardware
--------
- Rockchip RK3568 SoC
- Quad Cortex-A55 CPU
- Mail G52 GPU
- 1 TOPs NPU
- 2/4/8GB LPDDR4 RAM
- 1x eMMC connector
- 1x M.2 M Key (2-lane PCIe 3.0) supporting NVMe SSD
- 1x Micro SD card slot
- 1x 1000M Ethernet with PoE support (additional PoE HAT required)
- 1x USB 3.0 Type-A port (Host)
- 1x USB 3.0 Type-A port (OTG)
- 2x USB 2.0 Type-A ports (Host)
- 40 pin color GPIO header

[1] https://radxa.com/products/rock3/3a

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

Signed-off-by: FUKAUMI Naoki <naoki@radxa.com>
Link: https://github.com/openwrt/openwrt/pull/15985
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

package/boot/uboot-rockchip/Makefile

@@ -199,6 +199,13 @@ define U-Boot/radxa-e25-rk3568
     radxa_e25
 endef
 
+define U-Boot/rock-3a-rk3568
+  $(U-Boot/rk3568/Default)
+  NAME:=ROCK 3A
+  BUILD_DEVICES:= \
+    radxa_rock-3a
+endef
+
 UBOOT_TARGETS := \
   nanopc-t4-rk3399 \
   nanopi-r4s-rk3399 \
@@ -217,7 +224,8 @@ UBOOT_TARGETS := \
   bpi-r2-pro-rk3568 \
   nanopi-r5c-rk3568 \
   nanopi-r5s-rk3568 \
-  radxa-e25-rk3568
+  radxa-e25-rk3568 \
+  rock-3a-rk3568
 
 UBOOT_CONFIGURE_VARS += USE_PRIVATE_LIBGCC=yes