x86: dfi-bt700: Add xHCI USB support

Change from EHCI to xHCI on the DFI BayTrail SoM.

The xHCI USB hub is connected to an GPIO on the DFI BayTrail SoM. For
correct operation, it needs to get reset upon power-up. Otherwise it
may happen that the hub is not detected after a software reboot. This
patch also configures this GPIO in the dts for correct operation.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>