mlx4: Add board identifier and firmware version to sysctl

In last mlx4 update (r325841) we lost the sysctl to show the
firmware version for mlx4 devices.
Add both board identifier and firmware version under:
sys.device.mlx4_core0.hw sysctl node.

Approved by: hselasky (mentor)
MFC after: 1 week
Sponsored by: Mellanox Technologies