net: systemport: add Broadcom SYSTEMPORT Ethernet MAC driver

SYSTEMPORT is the latest Ethernet MAC hardware block used on newer
BCM7xxx Set Top Box SoCs in conjunction with an internal Ethernet
switch. This patch adds support for this hardware block along with the
following hardware features:

- support for hardware checksum offload (transmit and receive)
- support for the 32 transmit queues
- MIB counters reading

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>