syscon: dt-bindings: Add documentation for Aspeed system control units

Signed-off-by: NAndrew Jeffery <andrew@aj.id.au>
Acked-by: NRob Herring <robh@kernel.org>
Acked-by: NJoel Stanley <joel@jms.id.au>
Acked-by: NLinus Walleij <linus.walleij@linaro.org>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

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

+The Aspeed System Control Unit manages the global behaviour of the SoC,
+configuring elements such as clocks, pinmux, and reset.
+
+Required properties:
+- compatible:	One of:
+		"aspeed,ast2400-scu", "syscon", "simple-mfd"
+		"aspeed,g4-scu", "syscon", "simple-mfd"
+		"aspeed,ast2500-scu", "syscon", "simple-mfd"
+		"aspeed,g5-scu", "syscon", "simple-mfd"
+
+- reg:		contains the offset and length of the SCU memory region
+
+Example:
+
+syscon: syscon@1e6e2000 {
+	compatible = "aspeed,ast2400-scu", "syscon", "simple-mfd";
+	reg = <0x1e6e2000 0x1a8>;
+};