dt-bindings: Use full path to other schemas

When referencing other schema, it is preferred to use an absolute path
(/schemas/....), which allows also an seamless move of particular schema
out of Linux kernel to dtschema.

Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Acked-by: Miquel Raynal <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring (Arm) <[e-mail]>

Documentation/devicetree/bindings/input/azoteq,iqs7222.yaml

···
       gpio-2: GPIO4
 
     allOf:
-      - $ref: ../pinctrl/pincfg-node.yaml#
+      - $ref: /schemas/pinctrl/pincfg-node.yaml#
 
     properties:
       drive-open-drain: true

Documentation/devicetree/bindings/media/amphion,vpu.yaml

···
     description:
       Each vpu encoder or decoder correspond a MU, which used for communication
       between driver and firmware. Implement via mailbox on driver.
-    $ref: ../mailbox/fsl,mu.yaml#
+    $ref: /schemas/mailbox/fsl,mu.yaml#
 
 
   "^vpu-core@[0-9a-f]+$":

Documentation/devicetree/bindings/mtd/mtd.yaml

···
     type: object
 
     allOf:
-      - $ref: ../nvmem/nvmem.yaml#
-      - $ref: ../nvmem/nvmem-deprecated-cells.yaml#
+      - $ref: /schemas/nvmem/nvmem.yaml#
+      - $ref: /schemas/nvmem/nvmem-deprecated-cells.yaml#
 
     unevaluatedProperties: false