board: rockchip: Add Radxa ROCK S0

Radxa ROCK S0 is a single-board computer based on the Rockchip RK3308B
SoC in an ultra-compact form factor. Add a board target for the board.

Features tested on a ROCK S0 v1.2 with 512 MiB RAM and 8 GiB eMMC:
- 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>
diff --git a/board/rockchip/evb_rk3308/MAINTAINERS b/board/rockchip/evb_rk3308/MAINTAINERS
index abffbb1..cd219c6 100644
--- a/board/rockchip/evb_rk3308/MAINTAINERS
+++ b/board/rockchip/evb_rk3308/MAINTAINERS
@@ -12,3 +12,9 @@
 S:      Maintained
 F:      configs/rock-pi-s-rk3308_defconfig
 F:      arch/arm/dts/rk3308-rock-pi-s*
+
+ROCK-S0
+M:      Jonas Karlman <jonas@kwiboo.se>
+S:      Maintained
+F:      configs/rock-s0-rk3308_defconfig
+F:      arch/arm/dts/rk3308-rock-s0*