gianfar: Add paged allocation and Rx S/G

The eTSEC h/w is capable of scatter/gather on the receive side
too if MAXFRM > MRBLR, when the allowed maximum Rx frame size
is set to be greater than the maximum Rx buffer size (MRBLR).
It's about time the driver makes use of this h/w capability,
by supporting fixed buffer sizes and Rx S/G.

The buffer size given to eTSEC for reception is fixed to
1536B (must be multiple of 64), which is the same default
buffer size as before, used to accommodate standard MTU
(1500B) size frames.  As before, eTSEC can receive frames of
up to 9600B.  Individual Rx buffers are mapped to page halves
(page size for eTSEC systems is 4KB).  The skb is built around
the first buffer of a frame (using build_skb()).  In case the
frame spans multiple buffers, the trailing buffers are added
as Rx fragments to the skb.  The last buffer in frame is marked
by the L status flag.  A mechanism is in place to reuse the pages
owned by the driver (for Rx) for subsequent receptions.

Supporting fixed size buffers allows the implementation of Rx S/G,
which in turn removes the memory pressure issues the driver had
before when MTU was set for jumbo frame reception.
Also, in most cases, the Rx path becomes faster due to Rx page
reusal, since the overhead of allocating new rx buffers is removed
from the fast path.

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>