dt-bindings: arm: Convert nuvoton,npcm750 binding to YAML

The general trend is to have devicetree bindings in YAML format, to
allow automatic validation of bindings and devicetrees.

Convert the NPCM SoC family's binding to YAML before it accumulates more
entries.

The nuvoton,npcm750-evb compatible string is introduced to keep the
structure of the binding a little simpler.

Signed-off-by: Jonathan Neuschäfer <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Joel Stanley <[e-mail]>

Documentation/devicetree/bindings/arm/npcm/npcm.txt

···
-NPCM Platforms Device Tree Bindings
------------------------------------
-NPCM750 SoC
-Required root node properties:
-	- compatible = "nuvoton,npcm750";
-

Documentation/devicetree/bindings/arm/npcm/npcm.yaml

···
+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/arm/npcm/npcm.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: NPCM Platforms Device Tree Bindings
+
+maintainers:
+  - Jonathan Neuschäfer <[e-mail]>
+
+properties:
+  $nodename:
+    const: '/'
+  compatible:
+    oneOf:
+      - description: NPCM750 based boards
+        items:
+          - enum:
+              - nuvoton,npcm750-evb         # NPCM750 evaluation board
+          - const: nuvoton,npcm750
+
+additionalProperties: true