dt-bindings: usb: Add RK3588 EHCI

Add compatible for RK3588 EHCI. As far as I know it's fully
compatible with generic-ehci.

Acked-by: Krzysztof Kozlowski <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Sebastian Reichel <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

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

···
               - ibm,476gtr-ehci
               - nxp,lpc1850-ehci
               - qca,ar7100-ehci
+              - rockchip,rk3588-ehci
               - snps,hsdk-v1.0-ehci
               - socionext,uniphier-ehci
           - const: generic-ehci