Documentation: dt-bindings: update the meson-usb2-phy example

Update the example so the node name uses a dash (instead of an
underscore) as per convention.
Additionally it updates the example register offset to a real example
(the old value was taken from a draft where there was an additional PHY
bus).

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>