net: ethernet: bgmac: add NS2 support

Add support for the variant of amac hardware present in the Broadcom
Northstar2 based SoCs.  Northstar2 requires an additional register to be
configured with the port speed/duplexity (NICPM).  This can be added to
the link callback to hide it from the instances that do not use this.
Also, clearing of the pending interrupts on init is required due to
observed issues on some platforms.

Signed-off-by: Jon Mason <jon.mason@broadcom.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>