dt-bindings: vexpress-sysreg: Allow for no child nodes

It is valid for arm,vexpress-sysreg to have no child nodes when either
the deprecated binding without child nodes is used or a platform
has no GPIOs. If there are no child nodes, then "#address-cells" and
"#size-cells" should not be required. However, "reg" should be present
in all cases, so let's add it now.

Cc: Andre Przywara <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/devicetree/bindings/arm/vexpress-sysreg.yaml

···
 
 required:
   - compatible
-  - "#address-cells"
-  - "#size-cells"
+  - reg
 
 examples:
   - |