USB: xHCI: Add stack support for xHCI

This adds stack layer for eXtensible Host Controller Interface
which facilitates use of USB 3.0 in host mode.

Adapting xHCI host controller driver in linux-kernel
by Sarah Sharp to needs in u-boot.

Initial porting from Linux kernel version 3.4, with following
top commit history of drivers/usb/host/xhci* :
cf84055 xHCI: Cleanup isoc transfer ring when TD length mismatch found

This adds the basic xHCI host controller driver with bare minimum
features:
- Control/Bulk transfer support has been added with required
  infrastructure for necessary xHC data structures.
- Stream protocol hasn't been supported yet.
- No support for quirky devices has been added.

Signed-off-by: Vikas C Sajjan <vikas.sajjan@samsung.com>
Signed-off-by: Julius Werner <jwerner@chromium.org>
Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Dan Murphy <dmurphy@ti.com>
Cc: Marek Vasut <marex@denx.de>