dt-bindings: usb: generic-ehci: Add "companion" entry

"companion" entry is present in "generic.txt" usb binding file. This commit
adds it also in generic-ehci yaml binding.

Signed-off-by: Alexandre Torgue <[e-mail]>
[robh: restrict type to phandle instead of phandle-array]
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/usb/generic-ehci.yaml

···
     description:
       Set this flag to force EHCI reset after resume.
 
+  companion:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+     Phandle of a companion.
+
   phys: true
 
 required: