RDMA/bnxt_re: Skip backing store allocation for 57500 series

The backing store to keep HW context data structures is allocated and
initialized by L2 driver. For 57500 chip RoCE driver do not require to
allocate and initialize additional memory. Changing to skip duplicate
allocation and initialization for 57500 adapters. Driver continues as
before for older chips.

This patch also takes care of stats context memory alignment to 128
boundary, a requirement for 57500 series of chip. Older chips do not care
of alignment, thus the change is unconditional.

Signed-off-by: Selvin Xavier <selvin.xavier@broadcom.com>
Signed-off-by: Devesh Sharma <devesh.sharma@broadcom.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>