net: macb: Add support for jumbo frames

Enable jumbo frame support for Zynq Ultrascale+ MPSoC.
Update the NWCFG register and descriptor length masks accordingly.
Jumbo max length register should be set according to support in SoC; it is
set to 10240 for Zynq Ultrascale+ MPSoC.

Signed-off-by: Harini Katakam <harinik@xilinx.com>
Reviewed-by: Punnaiah Choudary Kalluri <punnaia@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>