usb: xhci: pci: Add GPIO support on the Etron EJ168 controllers

EJ168/EJ188/EJ198 are USB xHCI controllers. They also contain four GPIO
lines which are used on some systems to toggle an LED based on whether a
USB device is connected.

There is no public datasheet available for this hardware. All
information in this driver is taken from the
"F9K1115v2.03.97-GPL-10.2.85-20140313" GPL code dump of the Belkin
F9K1115v2. This board comes with an EJ168 USB xHCI controller and the
USB 3.0 LED is connected to one of the GPIOs. Inside the GPL source
archive the related code can be found in:
  linux/kernels/mips-linux-2.6.31/drivers/usb/host/etxhci-pci.c

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>