Documentation: devicetree: net: optional idm regs for bgmac

Specifying IDM register space in DT is not mendatory for SoCs
where firmware takes care of IDM operations. This patch updates
BGMAC driver's DT binding documentation indicating the same.

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