usb: xhci: Add reset controller support

Some atypical users of xhci might need to manually reset their xHCI
controller before starting the HCD setup. Check if a reset controller
device is available to the PCI bus and trigger a reset.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
[mb: squash fix to only build xhci_reset_hw() if CONFIG_DM_BUS]
Signed-off-by: Matthias Brugger <mbrugger@suse.com>