board: rockchip: Add FriendlyElec NanoPi R6S

The NanoPi R6S is a SBC by FriendlyElec based on the Rockchip RK3588s.
It comes with 4GB or 8GB of RAM, a microSD card slot, 32GB eMMC storage,
one RTL8211F 1GbE and two RTL8125 2.5GbE Ethernet ports, one USB 2.0
Type-A and one USB 3.0 Type-A port, a HDMI port, a 12-pin GPIO FPC
connector, a fan connector, IR receiver as well as some buttons and LEDs.

Add initial support for this board using the upstream devicetree sources.

Kernel commit:
f1b11f43b3e9 ("arm64: dts: rockchip: Add support for NanoPi R6S")

Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Sebastian Kropatsch <seb-dev@mail.de>