dt-bindings: aspeed: Add silicon id node to SCU

Different ASPEED families have various unique hardware silicon
identifiers within the SoC.
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20200921091644.133107-2-joel@jms.id.auSigned-off-by: Joel Stanley <joel@jms.id.au>

Documentation/devicetree/bindings/mfd/aspeed-scu.txt

@@ -20,3 +20,29 @@ syscon: syscon@1e6e2000 {
 	#clock-cells = <1>;
 	#reset-cells = <1>;
 };
+
+Silicon ID
+-----------------
+
+Families have unique hardware silicon identifiers within the SoC.
+
+Required properties:
+
+ - compatible:		"aspeed,silicon-id" or:
+			"aspeed,ast2400-silicon-id" or
+			"aspeed,ast2500-silicon-id" or
+			"aspeed,ast2600-silicon-id"
+
+ - reg:			offset and length of the silicon id information
+			optionally, a second offset and length describes the unique chip id
+
+			The reg should be the unique silicon id register, and
+			not backwards compatible one in eg. the 2600.
+
+Example:
+
+
+silicon-id@7c {
+        compatible = "aspeed,ast2500-silicon-id", "aspeed,silicon-id";
+        reg = <0x7c 0x4 0x150 0x8>;
+};