net/ncsi: Add NCSI OEM command support

This patch adds OEM commands and response handling. It also defines OEM
command and response structure as per NCSI specification along with its
handlers.

ncsi_cmd_handler_oem: This is a generic command request handler for OEM
commands
ncsi_rsp_handler_oem: This is a generic response handler for OEM commands

Signed-off-by: Vijay Khemka <vijaykhemka@fb.com>
Reviewed-by: Justin Lee <justin.lee1@dell.com>
Reviewed-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>