drivers: net: xgene: Change ring manager to use function pointers

This is a preparatory patch for adding ethernet support for APM X-Gene
ethernet driver to work with ring manager v2.

Added xgene_ring_ops structure for storing chip specific ring manager
properties and functions.

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>