Revert "dt-bindings: phy: Add documentation for NSP USB3 PHY"

This reverts commit c8ca631f ("dt-bindings: phy: Add documentation
for NSP USB3 PHY") to match reverting commit adding the new PHY driver.
Please note we revert this commit before it reached stable release.

If new compatible string is needed it should be added to the existing
bcm-ns-usb3-phy.txt which already describes this PHY.

Acked-by: Jon Mason <jon.mason@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>