net: macb: Allocate valid memory for TX and RX BD prefetch

GEM version in ZynqMP and most versions greater than r1p07 supports
TX and RX BD prefetch. The number of BDs that can be prefetched is a
HW configurable parameter. For ZynqMP, this parameter is 4.

When GEM DMA is accessing the last BD in the ring, even before the
BD is processed and the WRAP bit is noticed, it will have prefetched
BDs outside the BD ring. These will not be processed but it is
necessary to have accessible memory after the last BD. Especially
in cases where SMMU is used, memory locations immediately after the
last BD may not have translation tables triggering HRESP errors. Hence
always allocate extra BDs to accommodate for prefetch.
The value of tx/rx bd prefetch for any given SoC version is:
2 ^ (corresponding field in design config 10 register).
(value of this field >= 1)

Added a capability flag so that older IP versions that do not have
DCFG10 or this prefetch capability are not affected.

Signed-off-by: Harini Katakam <harini.katakam@xilinx.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>