dt-bindings: syscon: Add DT bindings documentation for Allwinner syscon

This patch adds documentation for Device-Tree bindings for the
syscon present in allwinner devices.
Signed-off-by: NCorentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/misc/allwinner,syscon.txt

+* Allwinner sun8i system controller
+
+This file describes the bindings for the system controller present in
+Allwinner SoC H3, A83T and A64.
+The principal function of this syscon is to control EMAC PHY choice and
+config.
+
+Required properties for the system controller:
+- reg: address and length of the register for the device.
+- compatible: should be "syscon" and one of the following string:
+		"allwinner,sun8i-h3-system-controller"
+		"allwinner,sun50i-a64-system-controller"
+		"allwinner,sun8i-a83t-system-controller"
+
+Example:
+syscon: syscon@1c00000 {
+	compatible = "allwinner,sun8i-h3-system-controller", "syscon";
+	reg = <0x01c00000 0x1000>;
+};