Revert "dt-bindings: usb: mtk-xhci: Make all clocks required"

This reverts commit ebc4969ae125e65fdb563f66f4bfa7aec95f7eb4. That
commit was supposed to make the binding better reflect the MediaTek XHCI
hardware block by requiring all clocks to be present. But doing that
also causes too much noise in the devicetrees, since it requires
updating old MediaTek DTs to add clock handles for the fixed clocks, and
going forward every new clock added to the binding would require even
more updates.

The commit also didn't update the example to match the changes, causing
additional warnings.

Instead let's keep the clocks optional so that old devicetrees can keep
omitting the fixed clocks, and we'll just add the clocks as required on
new DTs.

Fixes: ebc4969ae125 ("dt-bindings: usb: mtk-xhci: Make all clocks required")
Reviewed-by: AngeloGioacchino Del Regno <[e-mail]>
Reviewed-by: Chunfeng Yun <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Nícolas F. R. A. Prado <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/usb/mediatek,mtk-xhci.yaml

···
     maxItems: 1
 
   clocks:
+    minItems: 1
     items:
       - description: Controller clock used by normal mode
       - description: Reference clock used by low power mode etc
···
       - description: controller clock
 
   clock-names:
+    minItems: 1
     items:
-      - const: sys_ck
+      - const: sys_ck  # required, the following ones are optional
       - const: ref_ck
       - const: mcu_ck
       - const: dma_ck