usb: host: xhci: Add support for usb core indexing

Unique usb core id is used to differentiate between
different usb controllers.

Change-Id: Ibd886f704e7ecedfbf035b8e5adea2852ddf25bf
[jackp@codeaurora.org: squashed with usb: host: Fix passing of
core-id property to xhci-plat]
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>