rockchip: add NanoPi R2S support

rockchip: add NanoPi R2S support

Hardware
--------
RockChip RK3328 ARM64 (4 cores)
1GB DDR4 RAM
2x 1000 Base-T
3 LEDs (LAN / WAN / SYS)
1 Button (Reset)
Micro-SD slot
USB 2.0 Port

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

MAC-address
-----------
The vendor code supports reading a MAC address from an EEPROM connected
via i2c0 of the SoC. The EEPROM (address 0x51) should contain the MAC
address in binary at offset 0xfa. However, my two units didn't come with
such an EEPROM soldered on. The EEPROM should be placed between the SoC
and the GPIO pins on the board. (U10)

Generating rendom MAC addresses works around this issue. Otherwise, all
boards running the same image have identical MAC addresses.

Signed-off-by: David Bauer <mail@david-bauer.net>