net/bnxt: declare generic ring structs

net/bnxt: declare generic ring structs

Declare generic ring structures and a free() function. These are
generic ring management functions which will be used to create Tx,
Rx and Completion rings in the subsequent patches, and tie them to
the HWRM managed ring resources.

This generic ring structure is shared all the ring types and tracks
the the host Buffer Descriptors (BDs) and the HWRM assigned ID.

Signed-off-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Signed-off-by: Stephen Hurd <stephen.hurd@broadcom.com>
Reviewed-by: David Christensen <david.christensen@broadcom.com>