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>
diff --git a/board/rockchip/evb_rk3568/MAINTAINERS b/board/rockchip/evb_rk3568/MAINTAINERS
index e5b0986..ba4884d 100644
--- a/board/rockchip/evb_rk3568/MAINTAINERS
+++ b/board/rockchip/evb_rk3568/MAINTAINERS
@@ -69,3 +69,10 @@
 F:	configs/rock-3a-rk3568_defconfig
 F:	arch/arm/dts/rk3568-rock-3a.dts
 F:	arch/arm/dts/rk3568-rock-3a-u-boot.dtsi
+
+ROCK-3C
+M:	Jonas Karlman <jonas@kwiboo.se>
+M:	Maxim Moskalets <maximmosk4@gmail.com>
+S:	Maintained
+F:	arch/arm/dts/rk3566-rock-3c-u-boot.dtsi
+F:	configs/rock-3c-rk3566_defconfig