dt/bindings: clk: Add YAML schemas for LS1028A Display Clock bindings

LS1028A has a clock domain PXLCLK0 used for provide pixel clocks to Display
output interface. Add a YAML schema for this.
Signed-off-by: Wen He <wen.he_1@nxp.com>
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lkml.kernel.org/r/20191213083402.35678-1-wen.he_1@nxp.comSigned-off-by: Stephen Boyd <sboyd@kernel.org>

Documentation/devicetree/bindings/clock/fsl,plldig.yaml

+# SPDX-License-Identifier: GPL-2.0
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/bindings/clock/fsl,plldig.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: NXP QorIQ Layerscape LS1028A Display PIXEL Clock Binding
+
+maintainers:
+  - Wen He <wen.he_1@nxp.com>
+
+description: |
+  NXP LS1028A has a clock domain PXLCLK0 used for the Display output
+  interface in the display core, as implemented in TSMC CLN28HPM PLL.
+  which generate and offers pixel clocks to Display.
+
+properties:
+  compatible:
+    const: fsl,ls1028a-plldig
+
+  reg:
+    maxItems: 1
+
+  '#clock-cells':
+    const: 0
+
+  fsl,vco-hz:
+     description: Optional for VCO frequency of the PLL in Hertz.
+        The VCO frequency of this PLL cannot be changed during runtime
+        only at startup. Therefore, the output frequencies are very
+        limited and might not even closely match the requested frequency.
+        To work around this restriction the user may specify its own
+        desired VCO frequency for the PLL.
+     minimum: 650000000
+     maximum: 1300000000
+     default: 1188000000
+
+required:
+  - compatible
+  - reg
+  - clocks
+  - '#clock-cells'
+
+examples:
+  # Display PIXEL Clock node:
+  - |
+    dpclk: clock-display@f1f0000 {
+        compatible = "fsl,ls1028a-plldig";
+        reg = <0x0 0xf1f0000 0x0 0xffff>;
+        #clock-cells = <0>;
+        clocks = <&osc_27m>;
+    };
+
+...