bnxt_en: Add support for L2 doorbell size.

Read the L2 doorbell size from the firmware and only map the portion
of the doorbell BAR for L2 use.  This will leave the remaining doorbell
BAR available for the RoCE driver to use.  The RoCE driver can map
the remaining portion as write-combining to support the push feature.

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