dt-bindings: phy: add bcm63xx-usbh bindings

Document BCM63xx USBH PHY bindings.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200720131209.1236590-2-noltari@gmail.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>