gianfar: Add Multiple group Support

This patch introduces multiple group support for etsec2.0
devices.

Multiple group support is provided by mapping the set of enabled
queues to different groups and then programming the per group
regsiters imask, ievent, rstat, tstat.

The queues corresponding to a group are indicated by programming
isrg (interrupt steering) registers.

Signed-off-by: Sandeep Gopalpet <Sandeep.Kumar@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>