xhci: Introduce xhci_init_driver()

Since the struct hc_driver is mostly the same across the xhci-pci,
xhci-plat, and the upcoming xhci-tegra driver, introduce the function
xhci_init_driver() which will populate the hc_driver with the default
xHCI operations.  The caller must supply a setup function which will
be used as the hc_driver's reset callback.

Note that xhci-plat also overrides the default ->start() callback so
that it can do rcar-specific initialization.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>