bnxt_en: Re-structure doorbells.

The 57500 series chips have a new 64-bit doorbell format.  Use a new
bnxt_db_info structure to unify the new and the old 32-bit doorbells.
Add a new bnxt_set_db() function to set up the doorbell addreses and
doorbell keys ahead of time.  Modify and introduce new doorbell
helpers to help abstract and unify the old and new doorbells.

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>