dts: bindings: compatible properties are required

Any node which needs to generate defines from DTS must have a
"compatible" property, because the corresponding "constraint" is what
extract_dts_includes.py uses to match nodes with YAML files.

There are a few YAML files in the tree that list compatible as
"optional".  Fix them.

Signed-off-by: Marti Bolivar <marti@opensourcefoundries.com>
diff --git a/dts/bindings/arc/arc,dccm.yaml b/dts/bindings/arc/arc,dccm.yaml
index 7170df9..6cd1a79 100644
--- a/dts/bindings/arc/arc,dccm.yaml
+++ b/dts/bindings/arc/arc,dccm.yaml
@@ -14,7 +14,7 @@
 properties:
     compatible:
       type: string
-      category: optional
+      category: required
       description: compatible strings
       constraint: "arc,dccm"
 
diff --git a/dts/bindings/arc/arc,iccm.yaml b/dts/bindings/arc/arc,iccm.yaml
index aefbe72..5183e08 100644
--- a/dts/bindings/arc/arc,iccm.yaml
+++ b/dts/bindings/arc/arc,iccm.yaml
@@ -14,7 +14,7 @@
 properties:
     compatible:
       type: string
-      category: optional
+      category: required
       description: compatible strings
       constraint: "arc,iccm"
 
diff --git a/dts/bindings/mtd/soc-nv-flash.yaml b/dts/bindings/mtd/soc-nv-flash.yaml
index a85c6fd..5f31805 100644
--- a/dts/bindings/mtd/soc-nv-flash.yaml
+++ b/dts/bindings/mtd/soc-nv-flash.yaml
@@ -9,7 +9,7 @@
 properties:
     compatible:
       type: string
-      category: optional
+      category: required
       description: compatible strings
       constraint: "soc-nv-flash"