dt-bindings: add binding for the Allwinner A64 DE2 bus

All the sub-blocks of Allwinner A64 DE2 needs the SRAM C on A64 SoC to
be claimed, otherwise the whole DE2 space is inaccessible.

Add a device tree binding of the DE2 part as a sub-bus.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>

Documentation/devicetree/bindings/bus/sun50i-de2-bus.txt

+Device tree bindings for Allwinner A64 DE2 bus
+
+The Allwinner A64 DE2 is on a special bus, which needs a SRAM region (SRAM C)
+to be claimed for enabling the access.
+
+Required properties:
+
+ - compatible:		Should contain "allwinner,sun50i-a64-de2"
+ - reg:			A resource specifier for the register space
+ - #address-cells:	Must be set to 1
+ - #size-cells:		Must be set to 1
+ - ranges:		Must be set up to map the address space inside the
+			DE2, for the sub-blocks of DE2.
+ - allwinner,sram:	the SRAM that needs to be claimed
+
+Example:
+
+	de2@1000000 {
+		compatible = "allwinner,sun50i-a64-de2";
+		reg = <0x1000000 0x400000>;
+		allwinner,sram = <&de2_sram 1>;
+		#address-cells = <1>;
+		#size-cells = <1>;
+		ranges = <0 0x1000000 0x400000>;
+
+		display_clocks: clock@0 {
+			compatible = "allwinner,sun50i-a64-de2-clk";
+			reg = <0x0 0x100000>;
+			clocks = <&ccu CLK_DE>,
+				 <&ccu CLK_BUS_DE>;
+			clock-names = "mod",
+				      "bus";
+			resets = <&ccu RST_BUS_DE>;
+			#clock-cells = <1>;
+			#reset-cells = <1>;
+		};
+	};