48-bit DMA addressing is supported in NFP3800 HW and implemented
in NFDK firmware, so enable this feature in driver now. Note that
with this change, NFD3 firmware, which doesn't implement 48-bit
DMA, cannot be used for NFP3800 any more.

RX free list descriptor, used by both NFD3 and NFDK, is also modified
to support 48-bit DMA. That's OK because the top bits is always get
set to 0 when assigned with 40-bit address.

Based on initial work of Jakub Kicinski <jakub.kicinski@netronome.com>.
Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>