usb: xhci: Add helper APIs to return xhci dma addresses

dma address of secondary event ring, transfer ring and
device context base address are required to pass to
remote entity. Remote entity uses these addresses to
program xhci controller registers.

Change-Id: Ie0756d646a396a11b41b93e886bca9aff636ee5d
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>