riscv: dts: spacemit: add K3 Pico-ITX board support

K3 Pico-ITX is a 2.5-inch single-board computer equipted with a SpacemiT
K3 SoC.

This minimal device tree enables booting into a serial console with UART
output.

Signed-off-by: Guodong Xu <guodong@riscstar.com>
Link: https://lore.kernel.org/r/20260115-k3-basic-dt-v5-7-6990ac9f4308@riscstar.com
Signed-off-by: Yixun Lan <dlan@kernel.org>

Author: docularxu

arch/riscv/boot/dts/spacemit/Makefile

@@ -4,3 +4,4 @@ dtb-$(CONFIG_ARCH_SPACEMIT) += k1-milkv-jupiter.dtb
 dtb-$(CONFIG_ARCH_SPACEMIT) += k1-musepi-pro.dtb
 dtb-$(CONFIG_ARCH_SPACEMIT) += k1-orangepi-r2s.dtb
 dtb-$(CONFIG_ARCH_SPACEMIT) += k1-orangepi-rv2.dtb
+dtb-$(CONFIG_ARCH_SPACEMIT) += k3-pico-itx.dtb

arch/riscv/boot/dts/spacemit/k3-pico-itx.dts

@@ -0,0 +1,29 @@
+// SPDX-License-Identifier: (GPL-2.0 OR MIT)
+/*
+ * Copyright (c) 2026 SpacemiT (Hangzhou) Technology Co. Ltd
+ * Copyright (c) 2026 Guodong Xu <guodong@riscstar.com>
+ */
+
+#include "k3.dtsi"
+
+/ {
+	model = "SpacemiT K3 Pico-ITX";
+	compatible = "spacemit,k3-pico-itx", "spacemit,k3";
+
+	aliases {
+		serial0 = &uart0;
+	};
+
+	chosen {
+		stdout-path = "serial0";
+	};
+
+	memory@100000000 {
+		device_type = "memory";
+		reg = <0x1 0x00000000 0x4 0x00000000>;
+	};
+};
+
+&uart0 {
+	status = "okay";
+};