dt-bindings: usb: samsung,exynos-usb2: add missing required reg

"reg" property is required on Samsung S5PV210/Exynos EHCI/OHCI
controllers.

Fixes: 4bf2283cb208 ("dt-bindings: usb: samsung,exynos-usb2: convert to dtschema")
Reviewed-by: Alim Akhtar <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/usb/samsung,exynos-usb2.yaml

···
   - interrupts
   - phys
   - phy-names
+  - reg
 
 allOf:
   - $ref: usb-hcd.yaml#