board: rockchip: Add Radxa ROCK 3B

The Radxa ROCK 3B is a single-board computer based on the Pico-ITX form
factor (100mm x 75mm). Two versions of the ROCK 3B exists, a community
version based on the RK3568 SoC and an industrial version based on the
RK3568J SoC.

Features tested on ROCK 3B 8GB v1.51 (both variants):
- SD-card boot
- eMMC boot
- SPI Flash boot
- Ethernet
- PCIe/NVMe
- USB gadget
- USB host

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