This patch adds DM support to the xHCI PCI driver. Enabling its use
e.g. in x86 platforms.

Status: On the congatec BayTrail SoM, xHCI still does not work
correctly with this patch. Some internal timeouts lead to resets (BUG).
Additional work is needed here. I'm posting this version as WIP so that
other developers interested in this support might use it as a start.
I might get back to it in a few weeks as well.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: George McCollister <george.mccollister@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Marek Vasut <marex@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>