bcm63xx_enet: split DMA channel register accesses

The current bcm63xx_enet driver always uses bcmenet_shared_base whenever
it needs to access DMA channel configuration space or access the DMA
channel state RAM. Split these register in 3 parts to be more accurate:

- global DMA configuration
- per DMA channel configuration space
- per DMA channel state RAM space

This is preliminary to support new chips where the global DMA
configuration remains the same, but there is a varying number of DMA
channels located at a different memory offset.

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>