Reorganize the code for allocation and mapping of Doorbell
pages. Implements new HW command to get the BAR length used by L2
driver. These changes are used by the future patch which maps the WC
Doorbell pages.

Also, introduced a new lock dpi_tbl_lock for synchronize the DB page
allocation from users.

Link: https://lore.kernel.org/r/1686679943-17117-7-git-send-email-selvin.xavier@broadcom.comSigned-off-by: Selvin Xavier <selvin.xavier@broadcom.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>