board: rockchip: add Radxa ROCK 3 Model C

board: rockchip: add Radxa ROCK 3 Model C

Based on rock-3a-rk3568_defconfig.
Tested on v1.31 revision.

Board Specifications:
- Rockchip RK3566
- 1/2/4GB LPDDR4 2112MT/s
- eMMC socket
- uSD card slot
- M.2 2230 Connector
- GbE LAN with POE
- 3.5mm jack with mic
- HDMI 2.0, MIPI DSI/CSI
- USB 3.0 Host, USB 2.0 Host/OTG
- 40-pin GPIO expansion ports

Signed-off-by: Maxim Moskalets <maximmosk4@gmail.com>
Suggested-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Jonas Karlman <jonas@kwiboo.se>
Tested-by: FUKAUMI Naoki <naoki@radxa.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>