libibverbs: Extend support of NDR rates

NDR(106.25 Gbps) support exposed new data rates:
800 Gbps - NDR 8x.
1200 Gbps - NDR 12x.

Utility methods were updated to support the new rates mentioned above:

1. Rate to mult - Convert the IB rate enum to a multiple of 2.5 Gbps.
2. Rate to mbps - Convert IB rate enum to the mbps value.

In addition, speed_str() of ibv_devinfo was updated to consider the new
NDR rate.

Reference: IB Spec Release 1.5
PR: 285305
MFC after: 1 week
Sponsored by: NVidia networking

Change-Id: I77541e406f700585fbfeddc162d5a0e7b79a1c11
Signed-off-by: Slava Shwartsman <slavash@nvidia.com>