usb: host: xhci: add platform driver support

This adds a fairly simple xhci-platform driver support. Currently it is
used by the dwc3 driver for supporting host mode.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>