ANDROID: usb: host: export additional xhci symbols for ring management

Export the following xhci symbols which will allow vendor modules
to perform their own ring management. These will be used to create
and manage additional secondary rings that are used to offload to a
separate coprocessor which interacts with the xHC controller.

xhci_ring_alloc
    - Allocate a struct xhci_ring.

xhci_alloc_erst
xhci_free_erst
    - Allocate and free event ring segment tables.

xhci_trb_virt_to_dma
    - Used to retrieve the DMA address of a TRB

xhci_ring_cmd_db
    - Notify the controller when a new command is issued

xhci_alloc_command
xhci_free_command
    - Allocate and free a struct xhci_command

xhci_queue_stop_endpoint
    - Issue a stop endpoint command to the controller

Bug: 182336717
Change-Id: I9d4c5884108e656e890aca8137d4ef580bcd7a12
Signed-off-by: Jack Pham <jackp@codeaurora.org>