usb: dwc3: host: convey the PHYs to xhci

On some platforms a PHY may need to be handled also in the
host controller driver. Exynos5420 SoC requires some "PHY
tuning" based on the USB speed. This patch delivers dwc3's
PHYs to the xhci platform device when it's created.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Tested-by: Vivek Gautam <gautam.vivek@samsung.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>