drivers: net: xgene: Preparing for adding 10GbE support

- Rearranged code to pave the way for adding 10GbE support
- Added mac_ops structure containing function pointers for mac specific functions
- Added port_ops structure containing function pointers for port specific functions

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