dt-bindings: riscv: spacemit: add K3 and Pico-ITX board bindings

Add DT binding documentation for the SpacemiT K3 SoC and the board Pico-ITX
which is a 2.5-inch single-board computer.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Yixun Lan <dlan@gentoo.org>
Signed-off-by: Guodong Xu <guodong@riscstar.com>
Link: https://lore.kernel.org/r/20260115-k3-basic-dt-v5-5-6990ac9f4308@riscstar.com
Signed-off-by: Yixun Lan <dlan@kernel.org>

Author: docularxu

Documentation/devicetree/bindings/riscv/spacemit.yaml

@@ -7,6 +7,7 @@ $schema: http://devicetree.org/meta-schemas/core.yaml#
 title: SpacemiT SoC-based boards
 
 maintainers:
+  - Guodong Xu <guodong@riscstar.com>
   - Yangyu Chen <cyy@cyyself.name>
   - Yixun Lan <dlan@gentoo.org>
 
@@ -26,6 +27,10 @@ properties:
               - xunlong,orangepi-r2s
               - xunlong,orangepi-rv2
           - const: spacemit,k1
+      - items:
+          - enum:
+              - spacemit,k3-pico-itx
+          - const: spacemit,k3
 
 additionalProperties: true