dt-bindings: misc: aspeed-p2a-ctrl: add support

Document the ast2400, ast2500 PCI-to-AHB bridge control driver bindings.
Signed-off-by: Patrick Venture <venture@google.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/misc/aspeed-p2a-ctrl.txt

+======================================================================
+Device tree bindings for Aspeed AST2400/AST2500 PCI-to-AHB Bridge Control Driver
+======================================================================
+
+The bridge is available on platforms with the VGA enabled on the Aspeed device.
+In this case, the host has access to a 64KiB window into all of the BMC's
+memory.  The BMC can disable this bridge.  If the bridge is enabled, the host
+has read access to all the regions of memory, however the host only has read
+and write access depending on a register controlled by the BMC.
+
+Required properties:
+===================
+
+ - compatible: must be one of:
+	- "aspeed,ast2400-p2a-ctrl"
+	- "aspeed,ast2500-p2a-ctrl"
+
+Optional properties:
+===================
+
+- memory-region: A phandle to a reserved_memory region to be used for the PCI
+		to AHB mapping
+
+The p2a-control node should be the child of a syscon node with the required
+property:
+
+- compatible : Should be one of the following:
+		"aspeed,ast2400-scu", "syscon", "simple-mfd"
+		"aspeed,g4-scu", "syscon", "simple-mfd"
+		"aspeed,ast2500-scu", "syscon", "simple-mfd"
+		"aspeed,g5-scu", "syscon", "simple-mfd"
+
+Example
+===================
+
+g4 Example
+----------
+
+syscon: scu@1e6e2000 {
+	compatible = "aspeed,ast2400-scu", "syscon", "simple-mfd";
+	reg = <0x1e6e2000 0x1a8>;
+
+	p2a: p2a-control {
+		compatible = "aspeed,ast2400-p2a-ctrl";
+		memory-region = <&reserved_memory>;
+	};
+};