gianfar: Introduce logical group support.

This patch introduces the group structure. The elements of this
structure are the interrupt lines, their corresponding names,
the register memory map.
The elements for this group are factored out from the gfar_private
structure. The introduction of group structure will help in
providing support for newer versions of etsec.

Currently, the support is present only for single group and
single tx/rx queues.

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