Hi, I just found these potential rk28xx datasheet download links:
瑞芯微 开发手册 RK28xx+DATASHEET+V0.2.pdf - 下载 - 共享资料
瑞芯推出RK2808芯片方案(附带芯片手册) ← NewMsg无线ARM开发平台 ← NewMsg无线通信
They use a silly coin based credit system and you can only download after registration and when you have some coins. Maybe someone of you can tear it down from one of these servers.