dt-bindings: phy: zynqmp-psgtr: fix USB phy name

PHY_TYPE_USB is undefined and was added as PHY_TYPE_USB2 and
PHY_TYPE_USB3 in 2fbbc96d (phy: Add PHY header file for DT x Driver
defines, 2014-11-04). Fix documentation to avoid misleading users.

Signed-off-by: Liam Beguin <lvb@xiphos.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/20211117003841.2030813-1-lvb@xiphos.com


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