board: rockchip: Add FriendlyElec NanoPi R2S Plus

The FriendlyElec NanoPi R2S Plus is a single-board computer based on
Rockchip RK3328 SoC. It features e.g. 1 GB DDR4 RAM, 32 GB eMMC,
SD-card, 2x GbE LAN, optional M.2 SDIO Wi-Fi and 2x USB 2.0 host.

Features tested on a NanoPi R2S Plus 2309:
- SD-card boot
- eMMC boot
- Ethernet
- USB gadget
- USB host

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
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