Add context numbers for HW elements in mlx5en(4).

To access the data, set sysctl dev.mce.N.conf.debug_stats to 1.
This enables the sysctl node dev.mce.N.hw_ctx_debug. Its content is
the mapping of each channel' number to used receive queue and associated
completion queue, set of the transmit queues numbers and corresponding
completion queues.

Trimmed example output:
channel 30 rq 188 cq 1085
channel 30 tc 0 sq 187 cq 1084
channel 31 rq 191 cq 1087
channel 31 tc 0 sq 190 cq 1086

MFC after: 1 week
Sponsored by: Mellanox Technologies