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/arch/arm/dts/rk3308-rock-s0-u-boot.dtsi b/arch/arm/dts/rk3308-rock-s0-u-boot.dtsi
new file mode 100644
index 0000000..84ca2ee
--- /dev/null
+++ b/arch/arm/dts/rk3308-rock-s0-u-boot.dtsi
@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+#include "rk3308-u-boot.dtsi"
+
+&emmc_pwren {
+	bootph-pre-ram;
+	bootph-some-ram;
+};
+
+&uart0 {
+	bootph-all;
+	clock-frequency = <24000000>;
+};
+
+&uart0_xfer {
+	bootph-all;
+};
+
+&vdd_core {
+	regulator-init-microvolt = <1015000>;
+};