net: ethernet: bgmac: Make IDM register space optional

IDM operations are usually one time ops and should be done in
firmware itself. Driver is not supposed to touch IDM registers.

However, for some SoCs', driver is performing IDM read/writes.
So this patch masks IDM operations in case firmware is taking
care of IDM operations.

Signed-off-by: Abhishek Shah <abhishek.shah@broadcom.com>
Reviewed-by: Oza Oza <oza.oza@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>