usb: dwc3: remove generic PHYs forwarding for XHCI device

Commit 08f871a3 ("usb: dwc3: host: convey the PHYs to xhci") added
forwarding of the generic PHYs from DWC3 core to the instantiated XHCI-plat
device. However XHCI(-plat) driver never gained support for generic PHYs,
thus the lookup added by that commit is never used. In meantime the commit
d64ff406 ("usb: dwc3: use bus->sysdev for DMA configuration")
incorrectly changed the device used for creating lookup, making the lookup
useless and generic PHYs inaccessible from XHCI device.

However since commit 178a0bce ("usb: core: hcd: integrate the PHY
wrapper into the HCD core") USB HCD already handles generic PHYs acquired
from the HCD's 'sysdev', which in this case is DWC3 core device. This means
that creating any custom lookup entries for XHCI driver is no longer needed
and can be simply removed.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>