octeontx2-af: Add support for CGX link management

CGX LMAC initialization, link status polling etc is done
by low level secure firmware. For link management this patch
adds a interface or communication mechanism between firmware
and this kernel CGX driver.

- Firmware interface specification is defined in cgx_fw_if.h.
- Support to send/receive commands/events to/form firmware.
- events/commands implemented
  * link up
  * link down
  * reading firmware version

Signed-off-by: Linu Cherian <lcherian@marvell.com>
Signed-off-by: Nithya Mani <nmani@marvell.com>
Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>