dt-bindings: soc: xilinx: Add DT bindings to xlnx_vcu driver

Add Device Tree binding document for logicoreIP. This logicoreIP
provides the isolation between the processing system and
programmable logic. Also provides the clock related information.
Signed-off-by: Dhaval Shah <dshah@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

Documentation/devicetree/bindings/soc/xilinx/xlnx,vcu.txt

+LogicoreIP designed compatible with Xilinx ZYNQ family.
+-------------------------------------------------------
+
+General concept
+---------------
+
+LogicoreIP design to provide the isolation between processing system
+and programmable logic. Also provides the list of register set to configure
+the frequency.
+
+Required properties:
+- compatible: shall be one of:
+	"xlnx,vcu"
+	"xlnx,vcu-logicoreip-1.0"
+- reg, reg-names: There are two sets of registers need to provide.
+	1. vcu slcr
+	2. Logicore
+	reg-names should contain name for the each register sequence.
+- clocks: phandle for aclk and pll_ref clocksource
+- clock-names: The identification string, "aclk", is always required for
+   the axi clock. "pll_ref" is required for pll.
+Example:
+
+	xlnx_vcu: vcu@a0040000 {
+		compatible = "xlnx,vcu-logicoreip-1.0";
+		reg = <0x0 0xa0040000 0x0 0x1000>,
+			 <0x0 0xa0041000 0x0 0x1000>;
+		reg-names = "vcu_slcr", "logicore";
+		clocks = <&si570_1>, <&clkc 71>;
+		clock-names = "pll_ref", "aclk";
+	};