dts: bindings: Have 'required: true/false' instead of 'category: ...'

The 'category: required/optional' setting for properties is just a
yes/no thing. Using a boolean makes it clearer, so have
'required: true/false' instead.

Print a clear error when 'category:' is used:

    edtlib.EDTError: please put 'required: true' instead of 'category:
    required' in 'properties: foo: ...' in
    test-bindings/sub-node-parent.yaml - 'category' has been removed

The old scripts in scripts/dts/ ignore this setting, and only print a
warning if 'category: required' in an inherited binding is changed to
'category: optional'. Remove that code, since the new scripts already
have the same check.

The replacement was done with

    git ls-files 'dts/bindings/*.yaml' | xargs sed -i \
        -e 's/category:\s*required/required: true/' \
        -e 's/category:\s*optional/required: false/'

dts/binding-template.yaml is updated as well.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>

dts/bindings/riscv/openisa,rv32m1-pcc.yaml

@@ -17,10 +17,10 @@ properties:
       constraint: "openisa,rv32m1-pcc"
 
     reg:
-      category: required
+      required: true
 
     label:
-      category: required
+      required: true
 
     "#clock-cells":
       const: 1