bcm63xx_enet: add support for Broadcom BCM63xx integrated gigabit switch

Newer Broadcom BCM63xx SoCs: 6328, 6362 and 6368 have an integrated switch
which needs to be driven slightly differently from the traditional
external switches. This patch introduces changes in arch/mips/bcm63xx in order
to:

- register a bcm63xx_enetsw driver instead of bcm63xx_enet driver
- update DMA channels configuration & state RAM base addresses
- add a new platform data configuration knob to define the number of
  ports per switch/device and force link on some ports
- define the required switch registers

On the driver side, the following changes are required:

- the switch ports need to be polled to ensure the link is up and
  running and RX/TX can properly work
- basic switch configuration needs to be performed for the switch to
  forward packets to the CPU
- update the MIB counters since the integrated

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>